Thread: BitmapHeapScan streaming read user and prelim refactoring

BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

14 February, 02:11:25

Hi,

Attached is a patch set which refactors BitmapHeapScan such that it
can use the streaming read API [1]. It also resolves the long-standing
FIXME in the BitmapHeapScan code suggesting that the skip fetch
optimization should be pushed into the table AMs. Additionally, it
moves table scan initialization to after the index scan and bitmap
initialization.

patches 0001-0002 are assorted cleanup needed later in the set.
patches 0003 moves the table scan initialization to after bitmap creation
patch 0004 is, I think, a bug fix. see [2].
patches 0005-0006 push the skip fetch optimization into the table AMs
patches 0007-0009 change the control flow of BitmapHeapNext() to match
that required by the streaming read API
patch 0010 is the streaming read code not yet in master
patch 0011 is the actual bitmapheapscan streaming read user.

patches 0001-0009 apply on top of master but 0010 and 0011 must be
applied on top of a commit before a 21d9c3ee4ef74e2 (until a rebased
version of the streaming read API is on the mailing list).

The caveat is that these patches introduce breaking changes to two
table AM functions for bitmapheapscan: table_scan_bitmap_next_block()
and table_scan_bitmap_next_tuple().

A TBMIterateResult used to be threaded through both of these functions
and used in BitmapHeapNext(). This patch set removes all references to
TBMIterateResults from BitmapHeapNext. Because the streaming read API
requires the callback to specify the next block, BitmapHeapNext() can
no longer pass a TBMIterateResult to table_scan_bitmap_next_block().

More subtly, table_scan_bitmap_next_block() used to return false if
there were no more visible tuples on the page or if the block that was
requested was not valid. With these changes,
table_scan_bitmap_next_block() will only return false when the bitmap
has been exhausted and the scan can end. In order to use the streaming
read API, the user must be able to request the blocks it needs without
requiring synchronous feedback per block. Thus, this table AM function
must change its meaning.

I think the way the patches are split up could be improved. I will
think more about this. There are also probably a few mistakes with
which comments are updated in which patches in the set.

- Melanie

[1]
https://www.postgresql.org/message-id/flat/CA%2BhUKGJkOiOCa%2Bmag4BF%2BzHo7qo%3Do9CFheB8%3Dg6uT5TUm2gkvA%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAAKRu_bxrXeZ2rCnY8LyeC2Ls88KpjWrQ%2BopUrXDRXdcfwFZGA%40mail.gmail.com

Thank you so much for this thorough review!!!!

On Wed, Feb 14, 2024 at 2:42 PM Andres Freund <andres@anarazel.de> wrote:
>
>
> On 2024-02-13 18:11:25 -0500, Melanie Plageman wrote:
>
> > From d6dd6eb21dcfbc41208f87d1d81ffe3960130889 Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Mon, 12 Feb 2024 18:50:29 -0500
> > Subject: [PATCH v1 03/11] BitmapHeapScan begin scan after bitmap setup
> >
> > There is no reason for table_beginscan_bm() to begin the actual scan of
> > the underlying table in ExecInitBitmapHeapScan(). We can begin the
> > underlying table scan after the index scan has been completed and the
> > bitmap built.
> >
> > The one use of the scan descriptor during initialization was
> > ExecBitmapHeapInitializeWorker(), which set the scan descriptor snapshot
> > with one from an array in the parallel state. This overwrote the
> > snapshot set in table_beginscan_bm().
> >
> > By saving that worker snapshot as a member in the BitmapHeapScanState
> > during initialization, it can be restored in table_beginscan_bm() after
> > returning from the table AM specific begin scan function.
>
> I don't understand what the point of passing two different snapshots to
> table_beginscan_bm() is. What does that even mean? Why can't we just use the
> correct snapshot initially?

Indeed. Honestly, it was an unlabeled TODO for me. I wasn't quite sure
how to get the same behavior as in master. Fixed in attached v2.

Now the parallel worker still restores and registers that snapshot in
ExecBitmapHeapInitializeWorker() and then saves it in the
BitmapHeapScanState. We then pass SO_TEMP_SNAPSHOT as an extra flag
(to set rs_flags) to table_beginscan_bm() if there is a parallel
worker snapshot saved in the BitmapHeapScanState.

> > From a3f62e4299663d418531ae61bb16ea39f0836fac Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Mon, 12 Feb 2024 19:03:24 -0500
> > Subject: [PATCH v1 04/11] BitmapPrefetch use prefetch block recheck for skip
> >  fetch
> >
> > Previously BitmapPrefetch() used the recheck flag for the current block
> > to determine whether or not it could skip prefetching the proposed
> > prefetch block. It makes more sense for it to use the recheck flag from
> > the TBMIterateResult for the prefetch block instead.
>
> I'd mention the commit that introduced the current logic and link to the
> the thread that you started about this.

Done

> > From d56be7741765d93002649ef912ef4b8256a5b9af Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Mon, 12 Feb 2024 19:04:48 -0500
> > Subject: [PATCH v1 05/11] Update BitmapAdjustPrefetchIterator parameter type
> >  to BlockNumber
> >
> > BitmapAdjustPrefetchIterator() only used the blockno member of the
> > passed in TBMIterateResult to ensure that the prefetch iterator and
> > regular iterator stay in sync. Pass it the BlockNumber only. This will
> > allow us to move away from using the TBMIterateResult outside of table
> > AM specific code.
>
> Hm - I'm not convinced this is a good direction - doesn't that arguably
> *increase* TAM awareness? Perhaps it doesn't make much sense to use bitmap
> heap scans in a TAM without blocks, but still.

This is removed in later commits and is an intermediate state to try
and move the TBMIterateResult out of BitmapHeapNext(). I can find
another way to achieve this if it is important.

> > From 202b16d3a381210e8dbee69e68a8310be8ee11d2 Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Mon, 12 Feb 2024 20:15:05 -0500
> > Subject: [PATCH v1 06/11] Push BitmapHeapScan skip fetch optimization into
> >  table AM
> >
> > This resolves the long-standing FIXME in BitmapHeapNext() which said that
> > the optmization to skip fetching blocks of the underlying table when
> > none of the column data was needed should be pushed into the table AM
> > specific code.
>
> Long-standing? Sure, it's old enough to walk, but we have FIXMEs that are old
> enough to drink, at least in some countries. :)

;) I've updated the commit message. Though it is longstanding in that
it predates Melanie + Postgres.

> > The table AM agnostic functions for prefetching still need to know if
> > skipping fetching is permitted for this scan. However, this dependency
> > will be removed when that prefetching code is removed in favor of the
> > upcoming streaming read API.
>
> > ---
> >  src/backend/access/heap/heapam.c          |  10 +++
> >  src/backend/access/heap/heapam_handler.c  |  29 +++++++
> >  src/backend/executor/nodeBitmapHeapscan.c | 100 ++++++----------------
> >  src/include/access/heapam.h               |   2 +
> >  src/include/access/tableam.h              |  17 ++--
> >  src/include/nodes/execnodes.h             |   6 --
> >  6 files changed, 74 insertions(+), 90 deletions(-)
> >
> > diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
> > index 707460a5364..7aae1ecf0a9 100644
> > --- a/src/backend/access/heap/heapam.c
> > +++ b/src/backend/access/heap/heapam.c
> > @@ -955,6 +955,8 @@ heap_beginscan(Relation relation, Snapshot snapshot,
> >       scan->rs_base.rs_flags = flags;
> >       scan->rs_base.rs_parallel = parallel_scan;
> >       scan->rs_strategy = NULL;       /* set in initscan */
> > +     scan->vmbuffer = InvalidBuffer;
> > +     scan->empty_tuples = 0;
>
> These don't follow the existing naming pattern for HeapScanDescData. While I
> explicitly dislike the practice of adding prefixes to struct members, I don't
> think mixing conventions within a single struct improves things.

I've updated the names. What does rs even stand for?

> I also think it'd be good to note in comments that the vm buffer currently is
> only used for bitmap heap scans, otherwise one might think they'd also be used
> for normal scans, where we don't need them, because of the page level flag.

Done.

> Also, perhaps worth renaming "empty_tuples" to something indicating that it's
> the number of empty tuples to be returned later? num_empty_tuples_pending or
> such? Or the current "return_empty_tuples".

Done.

> > @@ -1043,6 +1045,10 @@ heap_rescan(TableScanDesc sscan, ScanKey key, bool set_params,
> >       if (BufferIsValid(scan->rs_cbuf))
> >               ReleaseBuffer(scan->rs_cbuf);
> >
> > +     if (BufferIsValid(scan->vmbuffer))
> > +             ReleaseBuffer(scan->vmbuffer);
> > +     scan->vmbuffer = InvalidBuffer;
>
> It does not matter one iota here, but personally I prefer moving the write
> inside the if, as dirtying the cacheline after we just figured out whe

I've now followed this convention throughout my patchset in the places
where I noticed it.

> > diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
> > index 9372b49bfaa..c0fb06c9688 100644
> > --- a/src/backend/executor/nodeBitmapHeapscan.c
> > +++ b/src/backend/executor/nodeBitmapHeapscan.c
> > @@ -108,6 +108,7 @@ BitmapHeapNext(BitmapHeapScanState *node)
> >        */
> >       if (!node->initialized)
> >       {
> > +             bool can_skip_fetch;
> >               /*
> >                * We can potentially skip fetching heap pages if we do not need any
> >                * columns of the table, either for checking non-indexable quals or
>
> Pretty sure pgindent will move this around.

This is gone now, but I have pgindented all the commits so it
shouldn't be a problem again.

> > +++ b/src/include/access/tableam.h
> > @@ -62,6 +62,7 @@ typedef enum ScanOptions
> >
> >       /* unregister snapshot at scan end? */
> >       SO_TEMP_SNAPSHOT = 1 << 9,
> > +     SO_CAN_SKIP_FETCH = 1 << 10,
> >  }                    ScanOptions;
>
> Would be nice to add a comment explaining what this flag means.

Done.

> > From 500c84019b982a1e6c8b8dd40240c8510d83c287 Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Tue, 13 Feb 2024 10:05:04 -0500
> > Subject: [PATCH v1 07/11] BitmapHeapScan scan desc counts lossy and exact
> >  pages
> >
> > Future commits will remove the TBMIterateResult from BitmapHeapNext(),
> > pushing it into the table AM-specific code. So we will have to keep
> > track of the number of lossy and exact pages in the scan descriptor.
> > Doing this change to lossy/exact page counting in a separate commit just
> > simplifies the diff.
>
> > ---
> >  src/backend/access/heap/heapam.c          |  2 ++
> >  src/backend/access/heap/heapam_handler.c  |  9 +++++++++
> >  src/backend/executor/nodeBitmapHeapscan.c | 18 +++++++++++++-----
> >  src/include/access/relscan.h              |  4 ++++
> >  4 files changed, 28 insertions(+), 5 deletions(-)
> >
> > diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
> > index 7aae1ecf0a9..88b4aad5820 100644
> > --- a/src/backend/access/heap/heapam.c
> > +++ b/src/backend/access/heap/heapam.c
> > @@ -957,6 +957,8 @@ heap_beginscan(Relation relation, Snapshot snapshot,
> >       scan->rs_strategy = NULL;       /* set in initscan */
> >       scan->vmbuffer = InvalidBuffer;
> >       scan->empty_tuples = 0;
> > +     scan->rs_base.lossy_pages = 0;
> > +     scan->rs_base.exact_pages = 0;
> >
> >       /*
> >        * Disable page-at-a-time mode if it's not a MVCC-safe snapshot.
> > diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
> > index baba09c87c0..6e85ef7a946 100644
> > --- a/src/backend/access/heap/heapam_handler.c
> > +++ b/src/backend/access/heap/heapam_handler.c
> > @@ -2242,6 +2242,15 @@ heapam_scan_bitmap_next_block(TableScanDesc scan,
> >       Assert(ntup <= MaxHeapTuplesPerPage);
> >       hscan->rs_ntuples = ntup;
> >
> > +     /* Only count exact and lossy pages with visible tuples */
> > +     if (ntup > 0)
> > +     {
> > +             if (tbmres->ntuples >= 0)
> > +                     scan->exact_pages++;
> > +             else
> > +                     scan->lossy_pages++;
> > +     }
> > +
> >       return ntup > 0;
> >  }
> >
> > diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
> > index c0fb06c9688..19d115de06f 100644
> > --- a/src/backend/executor/nodeBitmapHeapscan.c
> > +++ b/src/backend/executor/nodeBitmapHeapscan.c
> > @@ -53,6 +53,8 @@
> >  #include "utils/spccache.h"
> >
> >  static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
> > +static inline void BitmapAccumCounters(BitmapHeapScanState *node,
> > +                                                                        TableScanDesc scan);
> >  static inline void BitmapDoneInitializingSharedState(ParallelBitmapHeapState *pstate);
> >  static inline void BitmapAdjustPrefetchIterator(BitmapHeapScanState *node,
> >                                                                                               BlockNumber blockno);
> > @@ -234,11 +236,6 @@ BitmapHeapNext(BitmapHeapScanState *node)
> >                               continue;
> >                       }
> >
> > -                     if (tbmres->ntuples >= 0)
> > -                             node->exact_pages++;
> > -                     else
> > -                             node->lossy_pages++;
> > -
> >                       /* Adjust the prefetch target */
> >                       BitmapAdjustPrefetchTarget(node);
> >               }
> > @@ -315,9 +312,20 @@ BitmapHeapNext(BitmapHeapScanState *node)
> >       /*
> >        * if we get here it means we are at the end of the scan..
> >        */
> > +     BitmapAccumCounters(node, scan);
> >       return ExecClearTuple(slot);
> >  }
> >
> > +static inline void
> > +BitmapAccumCounters(BitmapHeapScanState *node,
> > +                                     TableScanDesc scan)
> > +{
> > +     node->exact_pages += scan->exact_pages;
> > +     scan->exact_pages = 0;
> > +     node->lossy_pages += scan->lossy_pages;
> > +     scan->lossy_pages = 0;
> > +}
> > +
>
> I don't think this is quite right - you're calling BitmapAccumCounters() only
> when the scan doesn't return anything anymore, but there's no guarantee
> that'll ever be reached. E.g. a bitmap heap scan below a limit node.  I think
> this needs to be in a) ExecEndBitmapHeapScan() b) ExecReScanBitmapHeapScan()

The scan descriptor isn't available in ExecEnd/ReScanBitmapHeapScan().
So, if we count in the scan descriptor we can't accumulate into the
BitmapHeapScanState there. The reason to count in the scan descriptor
is that it is in the table AM where we know if we have a lossy or
exact page -- and we only have the scan descriptor not the
BitmapHeapScanState in the table AM.

I added a call to BitmapAccumCounters before the tuple is returned for
correctness in this version (not ideal, I realize). See below for
thoughts about what we could do instead.

> >  /*
> >   *   BitmapDoneInitializingSharedState - Shared state is initialized
> >   *
> > diff --git a/src/include/access/relscan.h b/src/include/access/relscan.h
> > index 521043304ab..b74e08dd745 100644
> > --- a/src/include/access/relscan.h
> > +++ b/src/include/access/relscan.h
> > @@ -40,6 +40,10 @@ typedef struct TableScanDescData
> >       ItemPointerData rs_mintid;
> >       ItemPointerData rs_maxtid;
> >
> > +     /* Only used for Bitmap table scans */
> > +     long            exact_pages;
> > +     long            lossy_pages;
> > +
> >       /*
> >        * Information about type and behaviour of the scan, a bitmask of members
> >        * of the ScanOptions enum (see tableam.h).
>
> I wonder if this really is the best place for the data to be accumulated. This
> requires the accounting to be implemented in each AM, which doesn't obviously
> seem required.  Why can't the accounting continue to live in
> nodeBitmapHeapscan.c, to be done after each table_scan_bitmap_next_block()
> call?

Yes, I would really prefer not to do it in the table AM. But, we only
count exact and lossy pages for which at least one or more tuples were
visible (change this and you'll see tests fail). So, we need to decide
if we are going to increment the counters somewhere where we have
access to that information. In the case of heap, that is really only
once I have the value of ntup in heapam_scan_bitmap_next_block(). To
get that information back out to BitmapHeapNext(), I considered adding
another parameter to heapam_scan_bitmap_next_block() -- maybe an enum
like this:

/*
 * BitmapHeapScans's bitmaps can choose to store per page information in a
 * lossy or exact way. Exact pages in the bitmap have the individual tuple
 * offsets that need to be visited while lossy pages in the bitmap have only the
 * block number of the page.
 */
typedef enum BitmapBlockResolution
{
    BITMAP_BLOCK_NO_VISIBLE,
    BITMAP_BLOCK_LOSSY,
    BITMAP_BLOCK_EXACT,
} BitmapBlockResolution;

which we then use to increment the counter. But while I was writing
this code, I found myself narrating in the comment that the reason
this had to be set inside of the table AM is that only the table AM
knows if it wants to count the block as lossy, exact, or not count it.
So, that made me question if it really should be in the
BitmapHeapScanState.

I also explored passing the table scan descriptor to
show_tidbitmap_info() -- but that had its own problems.

> > From 555743e4bc885609d20768f7f2990c6ba69b13a9 Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Tue, 13 Feb 2024 10:57:07 -0500
> > Subject: [PATCH v1 09/11] Make table_scan_bitmap_next_block() async friendly
> >
> > table_scan_bitmap_next_block() previously returned false if we did not
> > wish to call table_scan_bitmap_next_tuple() on the tuples on the page.
> > This could happen when there were no visible tuples on the page or, due
> > to concurrent activity on the table, the block returned by the iterator
> > is past the known end of the table.
>
> This sounds a bit like the block is actually past the end of the table,
> but in reality this happens if the block is past the end of the table as it
> was when the scan was started. Somehow that feels significant, but I don't
> really know why I think that.

I have tried to update the commit message to make it clearer. I was
actually wondering: now that we do table_beginscan_bm() in
BitmapHeapNext() instead of ExecInitBitmapHeapScan(), have we reduced
or eliminated the opportunity for this to be true? initscan() sets
rs_nblocks and that now happens much later.

> > diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
> > index 88b4aad5820..d8569373987 100644
> > --- a/src/backend/access/heap/heapam.c
> > +++ b/src/backend/access/heap/heapam.c
> > @@ -959,6 +959,8 @@ heap_beginscan(Relation relation, Snapshot snapshot,
> >       scan->empty_tuples = 0;
> >       scan->rs_base.lossy_pages = 0;
> >       scan->rs_base.exact_pages = 0;
> > +     scan->rs_base.shared_tbmiterator = NULL;
> > +     scan->rs_base.tbmiterator = NULL;
> >
> >       /*
> >        * Disable page-at-a-time mode if it's not a MVCC-safe snapshot.
> > @@ -1051,6 +1053,18 @@ heap_rescan(TableScanDesc sscan, ScanKey key, bool set_params,
> >               ReleaseBuffer(scan->vmbuffer);
> >       scan->vmbuffer = InvalidBuffer;
> >
> > +     if (scan->rs_base.rs_flags & SO_TYPE_BITMAPSCAN)
> > +     {
> > +             if (scan->rs_base.shared_tbmiterator)
> > +                     tbm_end_shared_iterate(scan->rs_base.shared_tbmiterator);
> > +
> > +             if (scan->rs_base.tbmiterator)
> > +                     tbm_end_iterate(scan->rs_base.tbmiterator);
> > +     }
> > +
> > +     scan->rs_base.shared_tbmiterator = NULL;
> > +     scan->rs_base.tbmiterator = NULL;
> > +
> >       /*
> >        * reinitialize scan descriptor
> >        */
>
> If every AM would need to implement this, perhaps this shouldn't be done here,
> but in generic code?

Fixed.

> > --- a/src/backend/access/heap/heapam_handler.c
> > +++ b/src/backend/access/heap/heapam_handler.c
> > @@ -2114,17 +2114,49 @@ heapam_estimate_rel_size(Relation rel, int32 *attr_widths,
> >
> >  static bool
> >  heapam_scan_bitmap_next_block(TableScanDesc scan,
> > -                                                       TBMIterateResult *tbmres)
> > +                                                       bool *recheck, BlockNumber *blockno)
> >  {
> >       HeapScanDesc hscan = (HeapScanDesc) scan;
> > -     BlockNumber block = tbmres->blockno;
> > +     BlockNumber block;
> >       Buffer          buffer;
> >       Snapshot        snapshot;
> >       int                     ntup;
> > +     TBMIterateResult *tbmres;
> >
> >       hscan->rs_cindex = 0;
> >       hscan->rs_ntuples = 0;
> >
> > +     *blockno = InvalidBlockNumber;
> > +     *recheck = true;
> > +
> > +     do
> > +     {
> > +             if (scan->shared_tbmiterator)
> > +                     tbmres = tbm_shared_iterate(scan->shared_tbmiterator);
> > +             else
> > +                     tbmres = tbm_iterate(scan->tbmiterator);
> > +
> > +             if (tbmres == NULL)
> > +             {
> > +                     /* no more entries in the bitmap */
> > +                     Assert(hscan->empty_tuples == 0);
> > +                     return false;
> > +             }
> > +
> > +             /*
> > +              * Ignore any claimed entries past what we think is the end of the
> > +              * relation. It may have been extended after the start of our scan (we
> > +              * only hold an AccessShareLock, and it could be inserts from this
> > +              * backend).  We don't take this optimization in SERIALIZABLE
> > +              * isolation though, as we need to examine all invisible tuples
> > +              * reachable by the index.
> > +              */
> > +     } while (!IsolationIsSerializable() && tbmres->blockno >= hscan->rs_nblocks);
>
> Hm. Isn't it a problem that we have no CHECK_FOR_INTERRUPTS() in this loop?

Yes. fixed.

> > @@ -2251,7 +2274,14 @@ heapam_scan_bitmap_next_block(TableScanDesc scan,
> >                       scan->lossy_pages++;
> >       }
> >
> > -     return ntup > 0;
> > +     /*
> > +      * Return true to indicate that a valid block was found and the bitmap is
> > +      * not exhausted. If there are no visible tuples on this page,
> > +      * hscan->rs_ntuples will be 0 and heapam_scan_bitmap_next_tuple() will
> > +      * return false returning control to this function to advance to the next
> > +      * block in the bitmap.
> > +      */
> > +     return true;
> >  }
>
> Why can't we fetch the next block immediately?

We don't know that we want another block until we've gone through this
page and seen there were no visible tuples, so we'd somehow have to
jump back up to the top of the function to get the next block -- which
is basically what is happening in my revised control flow. We call
heapam_scan_bitmap_next_tuple() and rs_ntuples is 0, so we end up
calling heapam_scan_bitmap_next_block() right away.

> > @@ -201,46 +197,23 @@ BitmapHeapNext(BitmapHeapScanState *node)
> >
                 can_skip_fetch); 
> >               }
> >
> > -             node->tbmiterator = tbmiterator;
> > -             node->shared_tbmiterator = shared_tbmiterator;
> > +             scan->tbmiterator = tbmiterator;
> > +             scan->shared_tbmiterator = shared_tbmiterator;
>
> It seems a bit odd that this code modifies the scan descriptor, instead of
> passing the iterator, or perhaps better the bitmap itself, to
> table_beginscan_bm()?

On rescan we actually will have initialized = false and make new
iterators but have the old scan descriptor. So, we need to be able to
set the iterator in the scan to the new iterator.

> > diff --git a/src/include/access/relscan.h b/src/include/access/relscan.h
> > index b74e08dd745..bf7ee044268 100644
> > --- a/src/include/access/relscan.h
> > +++ b/src/include/access/relscan.h
> > @@ -16,6 +16,7 @@
> >
> >  #include "access/htup_details.h"
> >  #include "access/itup.h"
> > +#include "nodes/tidbitmap.h"
>
> I'd like to avoid exposing this to everything including relscan.h. I think we
> could just forward declare the structs and use them here to avoid that?

Done

> > From aac60985d6bc70bfedf77a77ee3c512da87bfcb1 Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Tue, 13 Feb 2024 14:27:57 -0500
> > Subject: [PATCH v1 11/11] BitmapHeapScan uses streaming read API
> >
> > Remove all of the code to do prefetching from BitmapHeapScan code and
> > rely on the streaming read API prefetching. Heap table AM implements a
> > streaming read callback which uses the iterator to get the next valid
> > block that needs to be fetched for the streaming read API.
> > ---
> >  src/backend/access/gin/ginget.c           |  15 +-
> >  src/backend/access/gin/ginscan.c          |   7 +
> >  src/backend/access/heap/heapam.c          |  71 +++++
> >  src/backend/access/heap/heapam_handler.c  |  78 +++--
> >  src/backend/executor/nodeBitmapHeapscan.c | 328 +---------------------
> >  src/backend/nodes/tidbitmap.c             |  80 +++---
> >  src/include/access/heapam.h               |   2 +
> >  src/include/access/tableam.h              |  14 +-
> >  src/include/nodes/execnodes.h             |  19 --
> >  src/include/nodes/tidbitmap.h             |   8 +-
> >  10 files changed, 178 insertions(+), 444 deletions(-)
> >
> > diff --git a/src/backend/access/gin/ginget.c b/src/backend/access/gin/ginget.c
> > index 0b4f2ebadb6..3ce28078a6f 100644
> > --- a/src/backend/access/gin/ginget.c
> > +++ b/src/backend/access/gin/ginget.c
> > @@ -373,7 +373,10 @@ restartScanEntry:
> >                       if (entry->matchBitmap)
> >                       {
> >                               if (entry->matchIterator)
> > +                             {
> >                                       tbm_end_iterate(entry->matchIterator);
> > +                                     pfree(entry->matchResult);
> > +                             }
> >                               entry->matchIterator = NULL;
> >                               tbm_free(entry->matchBitmap);
> >                               entry->matchBitmap = NULL;
> > @@ -386,6 +389,7 @@ restartScanEntry:
> >               if (entry->matchBitmap && !tbm_is_empty(entry->matchBitmap))
> >               {
> >                       entry->matchIterator = tbm_begin_iterate(entry->matchBitmap);
> > +                     entry->matchResult = palloc0(TBM_ITERATE_RESULT_SIZE);
>
> Do we actually have to use palloc0? TBM_ITERATE_RESULT_SIZE ain't small, so
> zeroing all of it isn't free.

Tests actually did fail when I didn't use palloc0.

This code is different now though. There are a few new patches in v2
that 1) make the offsets array in the TBMIterateResult fixed size and
then this makes it possible to 2) make matchResult an inline member of
the GinScanEntry. I have a TODO in the code asking if setting blockno
in the TBMIterateResult to InvalidBlockNumber is sufficient
"resetting".

> > +static BlockNumber bitmapheap_pgsr_next_single(PgStreamingRead *pgsr, void *pgsr_private,
> > +                                                     void *per_buffer_data);
>
> Is it correct to have _single in the name here? Aren't we also using for
> parallel scans?

Right. I had a separate parallel version and then deleted it. This is now fixed.

> > +static BlockNumber
> > +bitmapheap_pgsr_next_single(PgStreamingRead *pgsr, void *pgsr_private,
> > +                                                     void *per_buffer_data)
> > +{
> > +     TBMIterateResult *tbmres = per_buffer_data;
> > +     HeapScanDesc hdesc = (HeapScanDesc) pgsr_private;
> > +
> > +     for (;;)
> > +     {
> > +             if (hdesc->rs_base.shared_tbmiterator)
> > +                     tbm_shared_iterate(hdesc->rs_base.shared_tbmiterator, tbmres);
> > +             else
> > +                     tbm_iterate(hdesc->rs_base.tbmiterator, tbmres);
> > +
> > +             /* no more entries in the bitmap */
> > +             if (!BlockNumberIsValid(tbmres->blockno))
> > +                     return InvalidBlockNumber;
> > +
> > +             /*
> > +              * Ignore any claimed entries past what we think is the end of the
> > +              * relation. It may have been extended after the start of our scan (we
> > +              * only hold an AccessShareLock, and it could be inserts from this
> > +              * backend).  We don't take this optimization in SERIALIZABLE
> > +              * isolation though, as we need to examine all invisible tuples
> > +              * reachable by the index.
> > +              */
> > +             if (!IsolationIsSerializable() && tbmres->blockno >= hdesc->rs_nblocks)
> > +                     continue;
> > +
> > +
> > +             if (hdesc->rs_base.rs_flags & SO_CAN_SKIP_FETCH &&
> > +                     !tbmres->recheck &&
> > +                     VM_ALL_VISIBLE(hdesc->rs_base.rs_rd, tbmres->blockno, &hdesc->vmbuffer))
> > +             {
> > +                     hdesc->empty_tuples += tbmres->ntuples;
> > +                     continue;
> > +             }
> > +
> > +             return tbmres->blockno;
> > +     }
> > +
> > +     /* not reachable */
> > +     Assert(false);
> > +}
>
> Need to check for interrupts somewhere here.

Done.

> > @@ -124,15 +119,6 @@ BitmapHeapNext(BitmapHeapScanState *node)
>
> There's still a comment in BitmapHeapNext talking about prefetching with two
> iterators etc. That seems outdated now.

Fixed.

> >  /*
> >   * tbm_iterate - scan through next page of a TIDBitmap
> >   *
> > - * Returns a TBMIterateResult representing one page, or NULL if there are
> > - * no more pages to scan.  Pages are guaranteed to be delivered in numerical
> > - * order.  If result->ntuples < 0, then the bitmap is "lossy" and failed to
> > - * remember the exact tuples to look at on this page --- the caller must
> > - * examine all tuples on the page and check if they meet the intended
> > - * condition.  If result->recheck is true, only the indicated tuples need
> > - * be examined, but the condition must be rechecked anyway.  (For ease of
> > - * testing, recheck is always set true when ntuples < 0.)
> > + * Caller must pass in a TBMIterateResult to be filled.
> > + *
> > + * Pages are guaranteed to be delivered in numerical order.  tbmres->blockno is
> > + * set to InvalidBlockNumber when there are no more pages to scan. If
> > + * tbmres->ntuples < 0, then the bitmap is "lossy" and failed to remember the
> > + * exact tuples to look at on this page --- the caller must examine all tuples
> > + * on the page and check if they meet the intended condition.  If
> > + * tbmres->recheck is true, only the indicated tuples need be examined, but the
> > + * condition must be rechecked anyway.  (For ease of testing, recheck is always
> > + * set true when ntuples < 0.)
> >   */
> > -TBMIterateResult *
> > -tbm_iterate(TBMIterator *iterator)
> > +void
> > +tbm_iterate(TBMIterator *iterator, TBMIterateResult *tbmres)
>
> Hm - it seems a tad odd that we later have to find out if the scan is done
> iterating by checking if blockno is valid, when tbm_iterate already knew. But
> I guess the code would be a bit uglier if we needed the result of
> tbm_[shared_]iterate(), due to the two functions.

Yes.

> Right now ExecEndBitmapHeapScan() frees the tbm before it does table_endscan()
> - which seems problematic, as heap_endscan() will do stuff like
> tbm_end_iterate(), which imo shouldn't be called after the tbm has been freed,
> even if that works today.

I've flipped the order -- I end the scan then free the bitmap.

> It seems a bit confusing that your changes seem to treat
> BitmapHeapScanState->initialized as separate from ->scan, even though afaict
> scan should be NULL iff initialized is false and vice versa.

I thought so too, but it seems on rescan that the node->initialized is
set to false but the scan is reused. So, we want to only make a new
scan descriptor if it is truly the beginning of a new scan.

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

16 February, 20:35:59

In the attached v3, I've reordered the commits, updated some errant
comments, and improved the commit messages.

I've also made some updates to the TIDBitmap API that seem like a
clarity improvement to the API in general. These also reduce the diff
for GIN when separating the TBMIterateResult from the
TBM[Shared]Iterator. And these TIDBitmap API changes are now all in
their own commits (previously those were in the same commit as adding
the BitmapHeapScan streaming read user).

The three outstanding issues I see in the patch set are:
1) the lossy and exact page counters issue described in my previous
email
2) the TODO in the TIDBitmap API changes about being sure that setting
TBMIterateResult->blockno to InvalidBlockNumber is sufficient for
indicating an invalid TBMIterateResult (and an exhausted bitmap)
3) the streaming read API is not committed yet, so the last two patches
are not "done"

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

27 February, 04:50:28

On Fri, Feb 16, 2024 at 12:35:59PM -0500, Melanie Plageman wrote:
> In the attached v3, I've reordered the commits, updated some errant
> comments, and improved the commit messages.
> 
> I've also made some updates to the TIDBitmap API that seem like a
> clarity improvement to the API in general. These also reduce the diff
> for GIN when separating the TBMIterateResult from the
> TBM[Shared]Iterator. And these TIDBitmap API changes are now all in
> their own commits (previously those were in the same commit as adding
> the BitmapHeapScan streaming read user).
> 
> The three outstanding issues I see in the patch set are:
> 1) the lossy and exact page counters issue described in my previous
> email

I've resolved this. I added a new patch to the set which starts counting
even pages with no visible tuples toward lossy and exact pages. After an
off-list conversation with Andres, it seems that this omission in master
may not have been intentional.

Once we have only two types of pages to differentiate between (lossy and
exact [no longer have to care about "has no visible tuples"]), it is
easy enough to pass a "lossy" boolean paramater to
table_scan_bitmap_next_block(). I've done this in the attached v4.

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

27 February, 17:22:30

On Mon, Feb 26, 2024 at 08:50:28PM -0500, Melanie Plageman wrote:
> On Fri, Feb 16, 2024 at 12:35:59PM -0500, Melanie Plageman wrote:
> > In the attached v3, I've reordered the commits, updated some errant
> > comments, and improved the commit messages.
> > 
> > I've also made some updates to the TIDBitmap API that seem like a
> > clarity improvement to the API in general. These also reduce the diff
> > for GIN when separating the TBMIterateResult from the
> > TBM[Shared]Iterator. And these TIDBitmap API changes are now all in
> > their own commits (previously those were in the same commit as adding
> > the BitmapHeapScan streaming read user).
> > 
> > The three outstanding issues I see in the patch set are:
> > 1) the lossy and exact page counters issue described in my previous
> > email
> 
> I've resolved this. I added a new patch to the set which starts counting
> even pages with no visible tuples toward lossy and exact pages. After an
> off-list conversation with Andres, it seems that this omission in master
> may not have been intentional.
> 
> Once we have only two types of pages to differentiate between (lossy and
> exact [no longer have to care about "has no visible tuples"]), it is
> easy enough to pass a "lossy" boolean paramater to
> table_scan_bitmap_next_block(). I've done this in the attached v4.

Thomas posted a new version of the Streaming Read API [1], so here is a
rebased v5. This should make it easier to review as it can be applied on
top of master.

- Melanie

[1] https://www.postgresql.org/message-id/CA%2BhUKGJtLyxcAEvLhVUhgD4fMQkOu3PDaj8Qb9SR_UsmzgsBpQ%40mail.gmail.com

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

28 February, 16:22:29

Hi,

I haven't looked at the code very closely yet, but I decided to do some
basic benchmarks to see if/how this refactoring affects behavior.

Attached is a simple .sh script that

1) creates a table with one of a couple basic data distributions
(uniform, linear, ...), with an index on top

2) runs a simple query with a where condition matching a known fraction
of the table (0 - 100%), and measures duration

3) the query is forced to use bitmapscan by disabling other options

4) there's a couple parameters the script varies (work_mem, parallel
workers, ...), the script drops caches etc.

5) I only have results for table with 1M rows, which is ~320MB, so not
huge. I'm running this for larger data set, but that will take time.


I did this on my two "usual" machines - i5 and xeon. Both have flash
storage, although i5 is SATA and xeon has NVMe. I won't share the raw
results, because the CSV is like 5MB - ping me off-list if you need the
file, ofc.

Attached is PDF summarizing the results as a pivot table, with results
for "master" and "patched" builds. The interesting bit is the last
column, which shows whether the patch makes it faster (green) or slower
(red).

The results seem pretty mixed, on both machines. If you focus on the
uncached results (pages 4 and 8-9), there's both runs that are much
faster (by a factor of 2-5x) and slower (similar factor).

Of course, these results are with forced bitmap scans, so the question
is if those regressions even matter - maybe we'd use a different scan
type, making these changes less severe. So I logged "optimal plan" for
each run, tracking the scan type the optimizer would really pick without
all the enable_* GUCs. And the -optimal.pdf shows only results for the
runs where the optimal plan uses the bitmap scan. And yes, while the
impact of the changes (in either direction) is reduced, it's still very
much there.

What's a bit surprising to me is that these regressions affect runs with
effective_io_concurrency=0 in particular, which traditionally meant to
not do any prefetching / async stuff. I've perceived the patch mostly as
refactoring, so have not really expected such massive impact on these cases.

So I wonder if the refactoring means that we're actually doing some sort
amount of prefetching even with e_i_c=0. I'm not sure that'd be great, I
assume people have valid reasons to disable prefetching ...


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On 2/28/24 15:56, Tomas Vondra wrote:
>> ...
> 
> Sure, I can do that. It'll take a couple hours to get the results, I'll
> share them when I have them.
> 

Here are the results with only patches 0001 - 0012 applied (i.e. without
the patch introducing the streaming read API, and the patch switching
the bitmap heap scan to use it).

The changes in performance don't disappear entirely, but the scale is
certainly much smaller - both in the complete results for all runs, and
for the "optimal" runs that would actually pick bitmapscan.

FWIW I'm not implying the patch must 100% maintain the current behavior,
or anything like that. At this point I'm more motivated to understand if
this change in behavior is expected and/or what this means for users.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


On 2/29/24 00:40, Melanie Plageman wrote:
> On Wed, Feb 28, 2024 at 6:17 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>>
>>
>> On 2/28/24 21:06, Melanie Plageman wrote:
>>> On Wed, Feb 28, 2024 at 2:23 PM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>
>>>> On 2/28/24 15:56, Tomas Vondra wrote:
>>>>>> ...
>>>>>
>>>>> Sure, I can do that. It'll take a couple hours to get the results, I'll
>>>>> share them when I have them.
>>>>>
>>>>
>>>> Here are the results with only patches 0001 - 0012 applied (i.e. without
>>>> the patch introducing the streaming read API, and the patch switching
>>>> the bitmap heap scan to use it).
>>>>
>>>> The changes in performance don't disappear entirely, but the scale is
>>>> certainly much smaller - both in the complete results for all runs, and
>>>> for the "optimal" runs that would actually pick bitmapscan.
>>>
>>> Hmm. I'm trying to think how my refactor could have had this impact.
>>> It seems like all the most notable regressions are with 4 parallel
>>> workers. What do the numeric column labels mean across the top
>>> (2,4,8,16...) -- are they related to "matches"? And if so, what does
>>> that mean?
>>>
>>
>> That's the number of distinct values matched by the query, which should
>> be an approximation of the number of matching rows. The number of
>> distinct values in the data set differs by data set, but for 1M rows
>> it's roughly like this:
>>
>> uniform: 10k
>> linear: 10k
>> cyclic: 100
>>
>> So for example matches=128 means ~1% of rows for uniform/linear, and
>> 100% for cyclic data sets.
> 
> Ah, thank you for the explanation. I also looked at your script after
> having sent this email and saw that it is clear in your script what
> "matches" is.
> 
>> As for the possible cause, I think it's clear most of the difference
>> comes from the last patch that actually switches bitmap heap scan to the
>> streaming read API. That's mostly expected/understandable, although we
>> probably need to look into the regressions or cases with e_i_c=0.
> 
> Right, I'm mostly surprised about the regressions for patches 0001-0012.
> 
> Re eic 0: Thomas Munro and I chatted off-list, and you bring up a
> great point about eic 0. In old bitmapheapscan code eic 0 basically
> disabled prefetching but with the streaming read API, it will still
> issue fadvises when eic is 0. That is an easy one line fix. Thomas
> prefers to fix it by always avoiding an fadvise for the last buffer in
> a range before issuing a read (since we are about to read it anyway,
> best not fadvise it too). This will fix eic 0 and also cut one system
> call from each invocation of the streaming read machinery.
> 
>> To analyze the 0001-0012 patches, maybe it'd be helpful to run tests for
>> individual patches. I can try doing that tomorrow. It'll have to be a
>> limited set of tests, to reduce the time, but might tell us whether it's
>> due to a single patch or multiple patches.
> 
> Yes, tomorrow I planned to start trying to repro some of the "red"
> cases myself. Any one of the commits could cause a slight regression
> but a 3.5x regression is quite surprising, so I might focus on trying
> to repro that locally and then narrow down which patch causes it.
> 
> For the non-cached regressions, perhaps the commit to use the correct
> recheck flag (0004) when prefetching could be the culprit. And for the
> cached regressions, my money is on the commit which changes the whole
> control flow of BitmapHeapNext() and the next_block() and next_tuple()
> functions (0010).
> 

I do have some partial results, comparing the patches. I only ran one of
the more affected workloads (cyclic) on the xeon, attached is a PDF
comparing master and the 0001-0014 patches. The percentages are timing
vs. the preceding patch (green - faster, red - slower).

This suggests only patches 0010 and 0014 affect performance, the rest is
just noise. I'll see if I can do more runs and get data from the other
machine (seems it's more significant on old SATA SSDs).

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

patch-comparison.pdf

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

01 March, 00:19:43

On Thu, Feb 29, 2024 at 7:54 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
>
>
> On 2/29/24 00:40, Melanie Plageman wrote:
> > On Wed, Feb 28, 2024 at 6:17 PM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >>
> >>
> >>
> >> On 2/28/24 21:06, Melanie Plageman wrote:
> >>> On Wed, Feb 28, 2024 at 2:23 PM Tomas Vondra
> >>> <tomas.vondra@enterprisedb.com> wrote:
> >>>>
> >>>> On 2/28/24 15:56, Tomas Vondra wrote:
> >>>>>> ...
> >>>>>
> >>>>> Sure, I can do that. It'll take a couple hours to get the results, I'll
> >>>>> share them when I have them.
> >>>>>
> >>>>
> >>>> Here are the results with only patches 0001 - 0012 applied (i.e. without
> >>>> the patch introducing the streaming read API, and the patch switching
> >>>> the bitmap heap scan to use it).
> >>>>
> >>>> The changes in performance don't disappear entirely, but the scale is
> >>>> certainly much smaller - both in the complete results for all runs, and
> >>>> for the "optimal" runs that would actually pick bitmapscan.
> >>>
> >>> Hmm. I'm trying to think how my refactor could have had this impact.
> >>> It seems like all the most notable regressions are with 4 parallel
> >>> workers. What do the numeric column labels mean across the top
> >>> (2,4,8,16...) -- are they related to "matches"? And if so, what does
> >>> that mean?
> >>>
> >>
> >> That's the number of distinct values matched by the query, which should
> >> be an approximation of the number of matching rows. The number of
> >> distinct values in the data set differs by data set, but for 1M rows
> >> it's roughly like this:
> >>
> >> uniform: 10k
> >> linear: 10k
> >> cyclic: 100
> >>
> >> So for example matches=128 means ~1% of rows for uniform/linear, and
> >> 100% for cyclic data sets.
> >
> > Ah, thank you for the explanation. I also looked at your script after
> > having sent this email and saw that it is clear in your script what
> > "matches" is.
> >
> >> As for the possible cause, I think it's clear most of the difference
> >> comes from the last patch that actually switches bitmap heap scan to the
> >> streaming read API. That's mostly expected/understandable, although we
> >> probably need to look into the regressions or cases with e_i_c=0.
> >
> > Right, I'm mostly surprised about the regressions for patches 0001-0012.
> >
> > Re eic 0: Thomas Munro and I chatted off-list, and you bring up a
> > great point about eic 0. In old bitmapheapscan code eic 0 basically
> > disabled prefetching but with the streaming read API, it will still
> > issue fadvises when eic is 0. That is an easy one line fix. Thomas
> > prefers to fix it by always avoiding an fadvise for the last buffer in
> > a range before issuing a read (since we are about to read it anyway,
> > best not fadvise it too). This will fix eic 0 and also cut one system
> > call from each invocation of the streaming read machinery.
> >
> >> To analyze the 0001-0012 patches, maybe it'd be helpful to run tests for
> >> individual patches. I can try doing that tomorrow. It'll have to be a
> >> limited set of tests, to reduce the time, but might tell us whether it's
> >> due to a single patch or multiple patches.
> >
> > Yes, tomorrow I planned to start trying to repro some of the "red"
> > cases myself. Any one of the commits could cause a slight regression
> > but a 3.5x regression is quite surprising, so I might focus on trying
> > to repro that locally and then narrow down which patch causes it.
> >
> > For the non-cached regressions, perhaps the commit to use the correct
> > recheck flag (0004) when prefetching could be the culprit. And for the
> > cached regressions, my money is on the commit which changes the whole
> > control flow of BitmapHeapNext() and the next_block() and next_tuple()
> > functions (0010).
> >
>
> I do have some partial results, comparing the patches. I only ran one of
> the more affected workloads (cyclic) on the xeon, attached is a PDF
> comparing master and the 0001-0014 patches. The percentages are timing
> vs. the preceding patch (green - faster, red - slower).

Just confirming: the results are for uncached?

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

01 March, 01:44:27


On 2/29/24 22:19, Melanie Plageman wrote:
> On Thu, Feb 29, 2024 at 7:54 AM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>>
>>
>> On 2/29/24 00:40, Melanie Plageman wrote:
>>> On Wed, Feb 28, 2024 at 6:17 PM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>
>>>>
>>>>
>>>> On 2/28/24 21:06, Melanie Plageman wrote:
>>>>> On Wed, Feb 28, 2024 at 2:23 PM Tomas Vondra
>>>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>>>
>>>>>> On 2/28/24 15:56, Tomas Vondra wrote:
>>>>>>>> ...
>>>>>>>
>>>>>>> Sure, I can do that. It'll take a couple hours to get the results, I'll
>>>>>>> share them when I have them.
>>>>>>>
>>>>>>
>>>>>> Here are the results with only patches 0001 - 0012 applied (i.e. without
>>>>>> the patch introducing the streaming read API, and the patch switching
>>>>>> the bitmap heap scan to use it).
>>>>>>
>>>>>> The changes in performance don't disappear entirely, but the scale is
>>>>>> certainly much smaller - both in the complete results for all runs, and
>>>>>> for the "optimal" runs that would actually pick bitmapscan.
>>>>>
>>>>> Hmm. I'm trying to think how my refactor could have had this impact.
>>>>> It seems like all the most notable regressions are with 4 parallel
>>>>> workers. What do the numeric column labels mean across the top
>>>>> (2,4,8,16...) -- are they related to "matches"? And if so, what does
>>>>> that mean?
>>>>>
>>>>
>>>> That's the number of distinct values matched by the query, which should
>>>> be an approximation of the number of matching rows. The number of
>>>> distinct values in the data set differs by data set, but for 1M rows
>>>> it's roughly like this:
>>>>
>>>> uniform: 10k
>>>> linear: 10k
>>>> cyclic: 100
>>>>
>>>> So for example matches=128 means ~1% of rows for uniform/linear, and
>>>> 100% for cyclic data sets.
>>>
>>> Ah, thank you for the explanation. I also looked at your script after
>>> having sent this email and saw that it is clear in your script what
>>> "matches" is.
>>>
>>>> As for the possible cause, I think it's clear most of the difference
>>>> comes from the last patch that actually switches bitmap heap scan to the
>>>> streaming read API. That's mostly expected/understandable, although we
>>>> probably need to look into the regressions or cases with e_i_c=0.
>>>
>>> Right, I'm mostly surprised about the regressions for patches 0001-0012.
>>>
>>> Re eic 0: Thomas Munro and I chatted off-list, and you bring up a
>>> great point about eic 0. In old bitmapheapscan code eic 0 basically
>>> disabled prefetching but with the streaming read API, it will still
>>> issue fadvises when eic is 0. That is an easy one line fix. Thomas
>>> prefers to fix it by always avoiding an fadvise for the last buffer in
>>> a range before issuing a read (since we are about to read it anyway,
>>> best not fadvise it too). This will fix eic 0 and also cut one system
>>> call from each invocation of the streaming read machinery.
>>>
>>>> To analyze the 0001-0012 patches, maybe it'd be helpful to run tests for
>>>> individual patches. I can try doing that tomorrow. It'll have to be a
>>>> limited set of tests, to reduce the time, but might tell us whether it's
>>>> due to a single patch or multiple patches.
>>>
>>> Yes, tomorrow I planned to start trying to repro some of the "red"
>>> cases myself. Any one of the commits could cause a slight regression
>>> but a 3.5x regression is quite surprising, so I might focus on trying
>>> to repro that locally and then narrow down which patch causes it.
>>>
>>> For the non-cached regressions, perhaps the commit to use the correct
>>> recheck flag (0004) when prefetching could be the culprit. And for the
>>> cached regressions, my money is on the commit which changes the whole
>>> control flow of BitmapHeapNext() and the next_block() and next_tuple()
>>> functions (0010).
>>>
>>
>> I do have some partial results, comparing the patches. I only ran one of
>> the more affected workloads (cyclic) on the xeon, attached is a PDF
>> comparing master and the 0001-0014 patches. The percentages are timing
>> vs. the preceding patch (green - faster, red - slower).
> 
> Just confirming: the results are for uncached?
> 

Yes, cyclic data set, uncached case. I picked this because it seemed
like one of the most affected cases. Do you want me to test some other
cases too?

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

01 March, 02:44:32

On 2/29/24 23:44, Tomas Vondra wrote:
>
> ...
> 
>>>
>>> I do have some partial results, comparing the patches. I only ran one of
>>> the more affected workloads (cyclic) on the xeon, attached is a PDF
>>> comparing master and the 0001-0014 patches. The percentages are timing
>>> vs. the preceding patch (green - faster, red - slower).
>>
>> Just confirming: the results are for uncached?
>>
> 
> Yes, cyclic data set, uncached case. I picked this because it seemed
> like one of the most affected cases. Do you want me to test some other
> cases too?
> 

BTW I decided to look at the data from a slightly different angle and
compare the behavior with increasing effective_io_concurrency. Attached
are charts for three "uncached" cases:

 * uniform, work_mem=4MB, workers_per_gather=0
 * linear-fuzz, work_mem=4MB, workers_per_gather=0
 * uniform, work_mem=4MB, workers_per_gather=4

Each page has charts for master and patched build (with all patches). I
think there's a pretty obvious difference in how increasing e_i_c
affects the two builds:

1) On master there's clear difference between eic=0 and eic=1 cases, but
on the patched build there's literally no difference - for example the
"uniform" distribution is clearly not great for prefetching, but eic=0
regresses to eic=1 poor behavior).

Note: This is where the the "red bands" in the charts come from.

2) For some reason, the prefetching with eic>1 perform much better with
the patches, except for with very low selectivity values (close to 0%).
Not sure why this is happening - either the overhead is much lower
(which would matter on these "adversarial" data distribution, but how
could that be when fadvise is not free), or it ends up not doing any
prefetching (but then what about (1)?).

3) I'm not sure about the linear-fuzz case, the only explanation I have
we're able to skip almost all of the prefetches (and read-ahead likely
works pretty well here).

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

prefetch-charts.pdf

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

01 March, 03:29:45

On Thu, Feb 29, 2024 at 5:44 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
>
>
> On 2/29/24 22:19, Melanie Plageman wrote:
> > On Thu, Feb 29, 2024 at 7:54 AM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >>
> >>
> >>
> >> On 2/29/24 00:40, Melanie Plageman wrote:
> >>> On Wed, Feb 28, 2024 at 6:17 PM Tomas Vondra
> >>> <tomas.vondra@enterprisedb.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2/28/24 21:06, Melanie Plageman wrote:
> >>>>> On Wed, Feb 28, 2024 at 2:23 PM Tomas Vondra
> >>>>> <tomas.vondra@enterprisedb.com> wrote:
> >>>>>>
> >>>>>> On 2/28/24 15:56, Tomas Vondra wrote:
> >>>>>>>> ...
> >>>>>>>
> >>>>>>> Sure, I can do that. It'll take a couple hours to get the results, I'll
> >>>>>>> share them when I have them.
> >>>>>>>
> >>>>>>
> >>>>>> Here are the results with only patches 0001 - 0012 applied (i.e. without
> >>>>>> the patch introducing the streaming read API, and the patch switching
> >>>>>> the bitmap heap scan to use it).
> >>>>>>
> >>>>>> The changes in performance don't disappear entirely, but the scale is
> >>>>>> certainly much smaller - both in the complete results for all runs, and
> >>>>>> for the "optimal" runs that would actually pick bitmapscan.
> >>>>>
> >>>>> Hmm. I'm trying to think how my refactor could have had this impact.
> >>>>> It seems like all the most notable regressions are with 4 parallel
> >>>>> workers. What do the numeric column labels mean across the top
> >>>>> (2,4,8,16...) -- are they related to "matches"? And if so, what does
> >>>>> that mean?
> >>>>>
> >>>>
> >>>> That's the number of distinct values matched by the query, which should
> >>>> be an approximation of the number of matching rows. The number of
> >>>> distinct values in the data set differs by data set, but for 1M rows
> >>>> it's roughly like this:
> >>>>
> >>>> uniform: 10k
> >>>> linear: 10k
> >>>> cyclic: 100
> >>>>
> >>>> So for example matches=128 means ~1% of rows for uniform/linear, and
> >>>> 100% for cyclic data sets.
> >>>
> >>> Ah, thank you for the explanation. I also looked at your script after
> >>> having sent this email and saw that it is clear in your script what
> >>> "matches" is.
> >>>
> >>>> As for the possible cause, I think it's clear most of the difference
> >>>> comes from the last patch that actually switches bitmap heap scan to the
> >>>> streaming read API. That's mostly expected/understandable, although we
> >>>> probably need to look into the regressions or cases with e_i_c=0.
> >>>
> >>> Right, I'm mostly surprised about the regressions for patches 0001-0012.
> >>>
> >>> Re eic 0: Thomas Munro and I chatted off-list, and you bring up a
> >>> great point about eic 0. In old bitmapheapscan code eic 0 basically
> >>> disabled prefetching but with the streaming read API, it will still
> >>> issue fadvises when eic is 0. That is an easy one line fix. Thomas
> >>> prefers to fix it by always avoiding an fadvise for the last buffer in
> >>> a range before issuing a read (since we are about to read it anyway,
> >>> best not fadvise it too). This will fix eic 0 and also cut one system
> >>> call from each invocation of the streaming read machinery.
> >>>
> >>>> To analyze the 0001-0012 patches, maybe it'd be helpful to run tests for
> >>>> individual patches. I can try doing that tomorrow. It'll have to be a
> >>>> limited set of tests, to reduce the time, but might tell us whether it's
> >>>> due to a single patch or multiple patches.
> >>>
> >>> Yes, tomorrow I planned to start trying to repro some of the "red"
> >>> cases myself. Any one of the commits could cause a slight regression
> >>> but a 3.5x regression is quite surprising, so I might focus on trying
> >>> to repro that locally and then narrow down which patch causes it.
> >>>
> >>> For the non-cached regressions, perhaps the commit to use the correct
> >>> recheck flag (0004) when prefetching could be the culprit. And for the
> >>> cached regressions, my money is on the commit which changes the whole
> >>> control flow of BitmapHeapNext() and the next_block() and next_tuple()
> >>> functions (0010).
> >>>
> >>
> >> I do have some partial results, comparing the patches. I only ran one of
> >> the more affected workloads (cyclic) on the xeon, attached is a PDF
> >> comparing master and the 0001-0014 patches. The percentages are timing
> >> vs. the preceding patch (green - faster, red - slower).
> >
> > Just confirming: the results are for uncached?
> >
>
> Yes, cyclic data set, uncached case. I picked this because it seemed
> like one of the most affected cases. Do you want me to test some other
> cases too?

So, I actually may have found the source of at least part of the
regression with 0010. I was able to reproduce the regression with
patch 0010 applied for the unached case with 4 workers and eic 8 and
100000000 rows for the cyclic dataset. I see it for all number of
matches. The regression went away (for this specific example) when I
moved the BitmapAdjustPrefetchIterator call back up to before the call
to table_scan_bitmap_next_block() like this:

diff --git a/src/backend/executor/nodeBitmapHeapscan.c
b/src/backend/executor/nodeBitmapHeapscan.c
index f7ecc060317..268996bdeea 100644
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -279,6 +279,8 @@ BitmapHeapNext(BitmapHeapScanState *node)
        }

 new_page:
+       BitmapAdjustPrefetchIterator(node, node->blockno);
+
        if (!table_scan_bitmap_next_block(scan, &node->recheck,
&lossy, &node->blockno))
            break;

@@ -287,7 +289,6 @@ new_page:
        else
            node->exact_pages++;

-       BitmapAdjustPrefetchIterator(node, node->blockno);
        /* Adjust the prefetch target */
        BitmapAdjustPrefetchTarget(node);
    }

It makes sense this would fix it. I haven't tried all the combinations
you tried. Do you mind running your tests with the new code? I've
pushed it into this branch.
https://github.com/melanieplageman/postgres/commits/bhs_pgsr/

Note that this will fix none of the issues with 0014 because that has
removed all of the old prefetching code anyway.

Thank you sooo much for running these to begin with and then helping
me figure out what is going on!

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

01 March, 04:18:20

On Thu, Feb 29, 2024 at 6:44 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 2/29/24 23:44, Tomas Vondra wrote:
> >
> > ...
> >
> >>>
> >>> I do have some partial results, comparing the patches. I only ran one of
> >>> the more affected workloads (cyclic) on the xeon, attached is a PDF
> >>> comparing master and the 0001-0014 patches. The percentages are timing
> >>> vs. the preceding patch (green - faster, red - slower).
> >>
> >> Just confirming: the results are for uncached?
> >>
> >
> > Yes, cyclic data set, uncached case. I picked this because it seemed
> > like one of the most affected cases. Do you want me to test some other
> > cases too?
> >
>
> BTW I decided to look at the data from a slightly different angle and
> compare the behavior with increasing effective_io_concurrency. Attached
> are charts for three "uncached" cases:
>
>  * uniform, work_mem=4MB, workers_per_gather=0
>  * linear-fuzz, work_mem=4MB, workers_per_gather=0
>  * uniform, work_mem=4MB, workers_per_gather=4
>
> Each page has charts for master and patched build (with all patches). I
> think there's a pretty obvious difference in how increasing e_i_c
> affects the two builds:

Wow! These visualizations make it exceptionally clear. I want to go to
the Vondra school of data visualizations for performance results!

> 1) On master there's clear difference between eic=0 and eic=1 cases, but
> on the patched build there's literally no difference - for example the
> "uniform" distribution is clearly not great for prefetching, but eic=0
> regresses to eic=1 poor behavior).

Yes, so eic=0 and eic=1 are identical with the streaming read API.
That is, eic 0 does not disable prefetching. Thomas is going to update
the streaming read API to avoid issuing an fadvise for the last block
in a range before issuing a read -- which would mean no prefetching
with eic 0 and eic 1. Not doing prefetching with eic 1 actually seems
like the right behavior -- which would be different than what master
is doing, right?

Hopefully this fixes the clear difference between master and the
patched version at eic 0.

> 2) For some reason, the prefetching with eic>1 perform much better with
> the patches, except for with very low selectivity values (close to 0%).
> Not sure why this is happening - either the overhead is much lower
> (which would matter on these "adversarial" data distribution, but how
> could that be when fadvise is not free), or it ends up not doing any
> prefetching (but then what about (1)?).

For the uniform with four parallel workers, eic == 0 being worse than
master makes sense for the above reason. But I'm not totally sure why
eic == 1 would be worse with the patch than with master. Both are
doing a (somewhat useless) prefetch.

With very low selectivity, you are less likely to get readahead
(right?) and similarly less likely to be able to build up > 8kB IOs --
which is one of the main value propositions of the streaming read
code. I imagine that this larger read benefit is part of why the
performance is better at higher selectivities with the patch. This
might be a silly experiment, but we could try decreasing
MAX_BUFFERS_PER_TRANSFER on the patched version and see if the
performance gains go away.

> 3) I'm not sure about the linear-fuzz case, the only explanation I have
> we're able to skip almost all of the prefetches (and read-ahead likely
> works pretty well here).

I started looking at the data generated by linear-fuzz to understand
exactly what effect the fuzz was having but haven't had time to really
understand the characteristics of this dataset. In the original
results, I thought uncached linear-fuzz and linear had similar results
(performance improvement from master). What do you expect with linear
vs linear-fuzz?

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

01 March, 17:05:49


On 3/1/24 02:18, Melanie Plageman wrote:
> On Thu, Feb 29, 2024 at 6:44 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 2/29/24 23:44, Tomas Vondra wrote:
>>>
>>> ...
>>>
>>>>>
>>>>> I do have some partial results, comparing the patches. I only ran one of
>>>>> the more affected workloads (cyclic) on the xeon, attached is a PDF
>>>>> comparing master and the 0001-0014 patches. The percentages are timing
>>>>> vs. the preceding patch (green - faster, red - slower).
>>>>
>>>> Just confirming: the results are for uncached?
>>>>
>>>
>>> Yes, cyclic data set, uncached case. I picked this because it seemed
>>> like one of the most affected cases. Do you want me to test some other
>>> cases too?
>>>
>>
>> BTW I decided to look at the data from a slightly different angle and
>> compare the behavior with increasing effective_io_concurrency. Attached
>> are charts for three "uncached" cases:
>>
>>  * uniform, work_mem=4MB, workers_per_gather=0
>>  * linear-fuzz, work_mem=4MB, workers_per_gather=0
>>  * uniform, work_mem=4MB, workers_per_gather=4
>>
>> Each page has charts for master and patched build (with all patches). I
>> think there's a pretty obvious difference in how increasing e_i_c
>> affects the two builds:
> 
> Wow! These visualizations make it exceptionally clear. I want to go to
> the Vondra school of data visualizations for performance results!
> 

Welcome to my lecture on how to visualize data. The process has about
four simple steps:

1) collect data for a lot of potentially interesting cases
2) load them into excel / google sheets / ...
3) slice and dice them into charts that you understand / can explain
4) every now and then there's something you can't understand / explain

Thank you for attending my lecture ;-) No homework today.

>> 1) On master there's clear difference between eic=0 and eic=1 cases, but
>> on the patched build there's literally no difference - for example the
>> "uniform" distribution is clearly not great for prefetching, but eic=0
>> regresses to eic=1 poor behavior).
> 
> Yes, so eic=0 and eic=1 are identical with the streaming read API.
> That is, eic 0 does not disable prefetching. Thomas is going to update
> the streaming read API to avoid issuing an fadvise for the last block
> in a range before issuing a read -- which would mean no prefetching
> with eic 0 and eic 1. Not doing prefetching with eic 1 actually seems
> like the right behavior -- which would be different than what master
> is doing, right?
> 

I don't think we should stop doing prefetching for eic=1, or at least
not based just on these charts. I suspect these "uniform" charts are not
a great example for the prefetching, because it's about distribution of
individual rows, and even a small fraction of rows may match most of the
pages. It's great for finding strange behaviors / corner cases, but
probably not a sufficient reason to change the default.

I think it makes sense to issue a prefetch one page ahead, before
reading/processing the preceding one, and it's fairly conservative
setting, and I assume the default was chosen for a reason / after
discussion.

My suggestion would be to keep the master behavior unless not practical,
and then maybe discuss changing the details later. The patch is already
complicated enough, better to leave that discussion for later.

> Hopefully this fixes the clear difference between master and the
> patched version at eic 0.
> 
>> 2) For some reason, the prefetching with eic>1 perform much better with
>> the patches, except for with very low selectivity values (close to 0%).
>> Not sure why this is happening - either the overhead is much lower
>> (which would matter on these "adversarial" data distribution, but how
>> could that be when fadvise is not free), or it ends up not doing any
>> prefetching (but then what about (1)?).
> 
> For the uniform with four parallel workers, eic == 0 being worse than
> master makes sense for the above reason. But I'm not totally sure why
> eic == 1 would be worse with the patch than with master. Both are
> doing a (somewhat useless) prefetch.
> 

Right.

> With very low selectivity, you are less likely to get readahead
> (right?) and similarly less likely to be able to build up > 8kB IOs --
> which is one of the main value propositions of the streaming read
> code. I imagine that this larger read benefit is part of why the
> performance is better at higher selectivities with the patch. This
> might be a silly experiment, but we could try decreasing
> MAX_BUFFERS_PER_TRANSFER on the patched version and see if the
> performance gains go away.
> 

Sure, I can do that. Do you have any particular suggestion what value to
use for MAX_BUFFERS_PER_TRANSFER?

I'll also try to add a better version of uniform, where the selectivity
matches more closely to pages, not rows.

>> 3) I'm not sure about the linear-fuzz case, the only explanation I have
>> we're able to skip almost all of the prefetches (and read-ahead likely
>> works pretty well here).
> 
> I started looking at the data generated by linear-fuzz to understand
> exactly what effect the fuzz was having but haven't had time to really
> understand the characteristics of this dataset. In the original
> results, I thought uncached linear-fuzz and linear had similar results
> (performance improvement from master). What do you expect with linear
> vs linear-fuzz?
> 

I don't know, TBH. My intent was to have a data set with correlated
data, either perfectly (linear) or with some noise (linear-fuzz). But
it's not like I spent too much thinking about it. It's more a case of
throwing stuff at the wall, seeing what sticks.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

01 March, 19:51:44

On Fri, Mar 1, 2024 at 9:05 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 3/1/24 02:18, Melanie Plageman wrote:
> > On Thu, Feb 29, 2024 at 6:44 PM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >>
> >> On 2/29/24 23:44, Tomas Vondra wrote:
> >> 1) On master there's clear difference between eic=0 and eic=1 cases, but
> >> on the patched build there's literally no difference - for example the
> >> "uniform" distribution is clearly not great for prefetching, but eic=0
> >> regresses to eic=1 poor behavior).
> >
> > Yes, so eic=0 and eic=1 are identical with the streaming read API.
> > That is, eic 0 does not disable prefetching. Thomas is going to update
> > the streaming read API to avoid issuing an fadvise for the last block
> > in a range before issuing a read -- which would mean no prefetching
> > with eic 0 and eic 1. Not doing prefetching with eic 1 actually seems
> > like the right behavior -- which would be different than what master
> > is doing, right?
>
> I don't think we should stop doing prefetching for eic=1, or at least
> not based just on these charts. I suspect these "uniform" charts are not
> a great example for the prefetching, because it's about distribution of
> individual rows, and even a small fraction of rows may match most of the
> pages. It's great for finding strange behaviors / corner cases, but
> probably not a sufficient reason to change the default.

Yes, I would like to see results from a data set where selectivity is
more correlated to pages/heap fetches. But, I'm not sure I see how
that is related to prefetching when eic = 1.

> I think it makes sense to issue a prefetch one page ahead, before
> reading/processing the preceding one, and it's fairly conservative
> setting, and I assume the default was chosen for a reason / after
> discussion.

Yes, I suppose the overhead of an fadvise does not compare to the IO
latency of synchronously reading that block. Actually, I bet the
regression I saw by accidentally moving BitmapAdjustPrefetchIterator()
after table_scan_bitmap_next_block() would be similar to the
regression introduced by making eic = 1 not prefetch.

When you think about IO concurrency = 1, it doesn't imply prefetching
to me. But, I think we want to do the right thing and have parity with
master.

> My suggestion would be to keep the master behavior unless not practical,
> and then maybe discuss changing the details later. The patch is already
> complicated enough, better to leave that discussion for later.

Agreed. Speaking of which, we need to add back use of tablespace IO
concurrency for the streaming read API (which is used by
BitmapHeapScan in master).

> > With very low selectivity, you are less likely to get readahead
> > (right?) and similarly less likely to be able to build up > 8kB IOs --
> > which is one of the main value propositions of the streaming read
> > code. I imagine that this larger read benefit is part of why the
> > performance is better at higher selectivities with the patch. This
> > might be a silly experiment, but we could try decreasing
> > MAX_BUFFERS_PER_TRANSFER on the patched version and see if the
> > performance gains go away.
>
> Sure, I can do that. Do you have any particular suggestion what value to
> use for MAX_BUFFERS_PER_TRANSFER?

I think setting it to 1 would be the same as always master -- doing
only 8kB reads. The only thing about that is that I imagine the other
streaming read code has some overhead which might end up being a
regression on balance even with the prefetching if we aren't actually
using the ranges/vectored capabilities of the streaming read
interface. Maybe if you just run it for one of the very obvious
performance improvement cases? I can also try this locally.

> I'll also try to add a better version of uniform, where the selectivity
> matches more closely to pages, not rows.

This would be great.

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

01 March, 20:08:03

On 3/1/24 17:51, Melanie Plageman wrote:
> On Fri, Mar 1, 2024 at 9:05 AM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 3/1/24 02:18, Melanie Plageman wrote:
>>> On Thu, Feb 29, 2024 at 6:44 PM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>
>>>> On 2/29/24 23:44, Tomas Vondra wrote:
>>>> 1) On master there's clear difference between eic=0 and eic=1 cases, but
>>>> on the patched build there's literally no difference - for example the
>>>> "uniform" distribution is clearly not great for prefetching, but eic=0
>>>> regresses to eic=1 poor behavior).
>>>
>>> Yes, so eic=0 and eic=1 are identical with the streaming read API.
>>> That is, eic 0 does not disable prefetching. Thomas is going to update
>>> the streaming read API to avoid issuing an fadvise for the last block
>>> in a range before issuing a read -- which would mean no prefetching
>>> with eic 0 and eic 1. Not doing prefetching with eic 1 actually seems
>>> like the right behavior -- which would be different than what master
>>> is doing, right?
>>
>> I don't think we should stop doing prefetching for eic=1, or at least
>> not based just on these charts. I suspect these "uniform" charts are not
>> a great example for the prefetching, because it's about distribution of
>> individual rows, and even a small fraction of rows may match most of the
>> pages. It's great for finding strange behaviors / corner cases, but
>> probably not a sufficient reason to change the default.
> 
> Yes, I would like to see results from a data set where selectivity is
> more correlated to pages/heap fetches. But, I'm not sure I see how
> that is related to prefetching when eic = 1.
> 

OK, I'll make that happen.

>> I think it makes sense to issue a prefetch one page ahead, before
>> reading/processing the preceding one, and it's fairly conservative
>> setting, and I assume the default was chosen for a reason / after
>> discussion.
> 
> Yes, I suppose the overhead of an fadvise does not compare to the IO
> latency of synchronously reading that block. Actually, I bet the
> regression I saw by accidentally moving BitmapAdjustPrefetchIterator()
> after table_scan_bitmap_next_block() would be similar to the
> regression introduced by making eic = 1 not prefetch.
> 
> When you think about IO concurrency = 1, it doesn't imply prefetching
> to me. But, I think we want to do the right thing and have parity with
> master.
> 

Just to be sure we're on the same page regarding what eic=1 means,
consider a simple sequence of pages: A, B, C, D, E, ...

With the current "master" code, eic=1 means we'll issue a prefetch for B
and then read+process A. And then issue prefetch for C and read+process
B, and so on. It's always one page ahead.

Yes, if the page is already in memory, the fadvise is just overhead. It
may happen for various reasons (say, read-ahead). But it's just this one
case, I'd bet in other cases eic=1 would be a win.

>> My suggestion would be to keep the master behavior unless not practical,
>> and then maybe discuss changing the details later. The patch is already
>> complicated enough, better to leave that discussion for later.
> 
> Agreed. Speaking of which, we need to add back use of tablespace IO
> concurrency for the streaming read API (which is used by
> BitmapHeapScan in master).
> 

+1

>>> With very low selectivity, you are less likely to get readahead
>>> (right?) and similarly less likely to be able to build up > 8kB IOs --
>>> which is one of the main value propositions of the streaming read
>>> code. I imagine that this larger read benefit is part of why the
>>> performance is better at higher selectivities with the patch. This
>>> might be a silly experiment, but we could try decreasing
>>> MAX_BUFFERS_PER_TRANSFER on the patched version and see if the
>>> performance gains go away.
>>
>> Sure, I can do that. Do you have any particular suggestion what value to
>> use for MAX_BUFFERS_PER_TRANSFER?
> 
> I think setting it to 1 would be the same as always master -- doing
> only 8kB reads. The only thing about that is that I imagine the other
> streaming read code has some overhead which might end up being a
> regression on balance even with the prefetching if we aren't actually
> using the ranges/vectored capabilities of the streaming read
> interface. Maybe if you just run it for one of the very obvious
> performance improvement cases? I can also try this locally.
> 

OK, I'll try with 1, and then we can adjust.

>> I'll also try to add a better version of uniform, where the selectivity
>> matches more closely to pages, not rows.
> 
> This would be great.
> 


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

01 March, 22:31:41

On Thu, Feb 29, 2024 at 7:29 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Thu, Feb 29, 2024 at 5:44 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
> >
> >
> >
> > On 2/29/24 22:19, Melanie Plageman wrote:
> > > On Thu, Feb 29, 2024 at 7:54 AM Tomas Vondra
> > > <tomas.vondra@enterprisedb.com> wrote:
> > >>
> > >>
> > >>
> > >> On 2/29/24 00:40, Melanie Plageman wrote:
> > >>> On Wed, Feb 28, 2024 at 6:17 PM Tomas Vondra
> > >>> <tomas.vondra@enterprisedb.com> wrote:
> > >>>>
> > >>>>
> > >>>>
> > >>>> On 2/28/24 21:06, Melanie Plageman wrote:
> > >>>>> On Wed, Feb 28, 2024 at 2:23 PM Tomas Vondra
> > >>>>> <tomas.vondra@enterprisedb.com> wrote:
> > >>>>>>
> > >>>>>> On 2/28/24 15:56, Tomas Vondra wrote:
> > >>>>>>>> ...
> > >>>>>>>
> > >>>>>>> Sure, I can do that. It'll take a couple hours to get the results, I'll
> > >>>>>>> share them when I have them.
> > >>>>>>>
> > >>>>>>
> > >>>>>> Here are the results with only patches 0001 - 0012 applied (i.e. without
> > >>>>>> the patch introducing the streaming read API, and the patch switching
> > >>>>>> the bitmap heap scan to use it).
> > >>>>>>
> > >>>>>> The changes in performance don't disappear entirely, but the scale is
> > >>>>>> certainly much smaller - both in the complete results for all runs, and
> > >>>>>> for the "optimal" runs that would actually pick bitmapscan.
> > >>>>>
> > >>>>> Hmm. I'm trying to think how my refactor could have had this impact.
> > >>>>> It seems like all the most notable regressions are with 4 parallel
> > >>>>> workers. What do the numeric column labels mean across the top
> > >>>>> (2,4,8,16...) -- are they related to "matches"? And if so, what does
> > >>>>> that mean?
> > >>>>>
> > >>>>
> > >>>> That's the number of distinct values matched by the query, which should
> > >>>> be an approximation of the number of matching rows. The number of
> > >>>> distinct values in the data set differs by data set, but for 1M rows
> > >>>> it's roughly like this:
> > >>>>
> > >>>> uniform: 10k
> > >>>> linear: 10k
> > >>>> cyclic: 100
> > >>>>
> > >>>> So for example matches=128 means ~1% of rows for uniform/linear, and
> > >>>> 100% for cyclic data sets.
> > >>>
> > >>> Ah, thank you for the explanation. I also looked at your script after
> > >>> having sent this email and saw that it is clear in your script what
> > >>> "matches" is.
> > >>>
> > >>>> As for the possible cause, I think it's clear most of the difference
> > >>>> comes from the last patch that actually switches bitmap heap scan to the
> > >>>> streaming read API. That's mostly expected/understandable, although we
> > >>>> probably need to look into the regressions or cases with e_i_c=0.
> > >>>
> > >>> Right, I'm mostly surprised about the regressions for patches 0001-0012.
> > >>>
> > >>> Re eic 0: Thomas Munro and I chatted off-list, and you bring up a
> > >>> great point about eic 0. In old bitmapheapscan code eic 0 basically
> > >>> disabled prefetching but with the streaming read API, it will still
> > >>> issue fadvises when eic is 0. That is an easy one line fix. Thomas
> > >>> prefers to fix it by always avoiding an fadvise for the last buffer in
> > >>> a range before issuing a read (since we are about to read it anyway,
> > >>> best not fadvise it too). This will fix eic 0 and also cut one system
> > >>> call from each invocation of the streaming read machinery.
> > >>>
> > >>>> To analyze the 0001-0012 patches, maybe it'd be helpful to run tests for
> > >>>> individual patches. I can try doing that tomorrow. It'll have to be a
> > >>>> limited set of tests, to reduce the time, but might tell us whether it's
> > >>>> due to a single patch or multiple patches.
> > >>>
> > >>> Yes, tomorrow I planned to start trying to repro some of the "red"
> > >>> cases myself. Any one of the commits could cause a slight regression
> > >>> but a 3.5x regression is quite surprising, so I might focus on trying
> > >>> to repro that locally and then narrow down which patch causes it.
> > >>>
> > >>> For the non-cached regressions, perhaps the commit to use the correct
> > >>> recheck flag (0004) when prefetching could be the culprit. And for the
> > >>> cached regressions, my money is on the commit which changes the whole
> > >>> control flow of BitmapHeapNext() and the next_block() and next_tuple()
> > >>> functions (0010).
> > >>>
> > >>
> > >> I do have some partial results, comparing the patches. I only ran one of
> > >> the more affected workloads (cyclic) on the xeon, attached is a PDF
> > >> comparing master and the 0001-0014 patches. The percentages are timing
> > >> vs. the preceding patch (green - faster, red - slower).
> > >
> > > Just confirming: the results are for uncached?
> > >
> >
> > Yes, cyclic data set, uncached case. I picked this because it seemed
> > like one of the most affected cases. Do you want me to test some other
> > cases too?
>
> So, I actually may have found the source of at least part of the
> regression with 0010. I was able to reproduce the regression with
> patch 0010 applied for the unached case with 4 workers and eic 8 and
> 100000000 rows for the cyclic dataset. I see it for all number of
> matches. The regression went away (for this specific example) when I
> moved the BitmapAdjustPrefetchIterator call back up to before the call
> to table_scan_bitmap_next_block() like this:
>
> diff --git a/src/backend/executor/nodeBitmapHeapscan.c
> b/src/backend/executor/nodeBitmapHeapscan.c
> index f7ecc060317..268996bdeea 100644
> --- a/src/backend/executor/nodeBitmapHeapscan.c
> +++ b/src/backend/executor/nodeBitmapHeapscan.c
> @@ -279,6 +279,8 @@ BitmapHeapNext(BitmapHeapScanState *node)
>         }
>
>  new_page:
> +       BitmapAdjustPrefetchIterator(node, node->blockno);
> +
>         if (!table_scan_bitmap_next_block(scan, &node->recheck,
> &lossy, &node->blockno))
>             break;
>
> @@ -287,7 +289,6 @@ new_page:
>         else
>             node->exact_pages++;
>
> -       BitmapAdjustPrefetchIterator(node, node->blockno);
>         /* Adjust the prefetch target */
>         BitmapAdjustPrefetchTarget(node);
>     }
>
> It makes sense this would fix it. I haven't tried all the combinations
> you tried. Do you mind running your tests with the new code? I've
> pushed it into this branch.
> https://github.com/melanieplageman/postgres/commits/bhs_pgsr/

Hold the phone on this one. I realized why I moved
BitmapAdjustPrefetchIterator after table_scan_bitmap_next_block() in
the first place -- master calls BitmapAdjustPrefetchIterator after the
tbm_iterate() for the current block -- otherwise with eic = 1, it
considers the prefetch iterator behind the current block iterator. I'm
going to go through and figure out what order this must be done in and
fix it.

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

02 March, 18:05:07

On 3/1/24 18:08, Tomas Vondra wrote:
> 
> On 3/1/24 17:51, Melanie Plageman wrote:
>> On Fri, Mar 1, 2024 at 9:05 AM Tomas Vondra
>> <tomas.vondra@enterprisedb.com> wrote:
>>>
>>> On 3/1/24 02:18, Melanie Plageman wrote:
>>>> On Thu, Feb 29, 2024 at 6:44 PM Tomas Vondra
>>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>>
>>>>> On 2/29/24 23:44, Tomas Vondra wrote:
>>>>> 1) On master there's clear difference between eic=0 and eic=1 cases, but
>>>>> on the patched build there's literally no difference - for example the
>>>>> "uniform" distribution is clearly not great for prefetching, but eic=0
>>>>> regresses to eic=1 poor behavior).
>>>>
>>>> Yes, so eic=0 and eic=1 are identical with the streaming read API.
>>>> That is, eic 0 does not disable prefetching. Thomas is going to update
>>>> the streaming read API to avoid issuing an fadvise for the last block
>>>> in a range before issuing a read -- which would mean no prefetching
>>>> with eic 0 and eic 1. Not doing prefetching with eic 1 actually seems
>>>> like the right behavior -- which would be different than what master
>>>> is doing, right?
>>>
>>> I don't think we should stop doing prefetching for eic=1, or at least
>>> not based just on these charts. I suspect these "uniform" charts are not
>>> a great example for the prefetching, because it's about distribution of
>>> individual rows, and even a small fraction of rows may match most of the
>>> pages. It's great for finding strange behaviors / corner cases, but
>>> probably not a sufficient reason to change the default.
>>
>> Yes, I would like to see results from a data set where selectivity is
>> more correlated to pages/heap fetches. But, I'm not sure I see how
>> that is related to prefetching when eic = 1.
>>
> 
> OK, I'll make that happen.
> 

Here's a PDF with charts for a dataset where the row selectivity is more
correlated to selectivity of pages. I'm attaching the updated script,
with the SQL generating the data set. But the short story is all rows on
a single page have the same random value, so the selectivity of rows and
pages should be the same.

The first page has results for the original "uniform", the second page
is the new "uniform-pages" data set. There are 4 charts, for
master/patched and 0/4 parallel workers. Overall the behavior is the
same, but for the "uniform-pages" it's much more gradual (with respect
to row selectivity). I think that's expected.

As for how this is related to eic=1 - I think my point was that these
are "adversary" data sets, most likely to show regressions. This applies
especially to the "uniform" data set, because as the row selectivity
grows, it's more and more likely it's right after to the current one,
and so a read-ahead would likely do the trick.

Also, this is forcing a bitmap scan plan - it's possible many of these
cases would use some other scan type, making the regression somewhat
irrelevant. Not entirely, because we make planning mistakes and for
robustness reasons it's good to keep the regression small.

But that's just how I think about it now. I don't think I have some
grand theory that'd dictate we have to do prefetching for eic=1.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


On 3/1/24 17:51, Melanie Plageman wrote:
> On Fri, Mar 1, 2024 at 9:05 AM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 3/1/24 02:18, Melanie Plageman wrote:
>>> On Thu, Feb 29, 2024 at 6:44 PM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>
>>>> On 2/29/24 23:44, Tomas Vondra wrote:
>>>> 1) On master there's clear difference between eic=0 and eic=1 cases, but
>>>> on the patched build there's literally no difference - for example the
>>>> "uniform" distribution is clearly not great for prefetching, but eic=0
>>>> regresses to eic=1 poor behavior).
>>>
>>> Yes, so eic=0 and eic=1 are identical with the streaming read API.
>>> That is, eic 0 does not disable prefetching. Thomas is going to update
>>> the streaming read API to avoid issuing an fadvise for the last block
>>> in a range before issuing a read -- which would mean no prefetching
>>> with eic 0 and eic 1. Not doing prefetching with eic 1 actually seems
>>> like the right behavior -- which would be different than what master
>>> is doing, right?
>>
>> I don't think we should stop doing prefetching for eic=1, or at least
>> not based just on these charts. I suspect these "uniform" charts are not
>> a great example for the prefetching, because it's about distribution of
>> individual rows, and even a small fraction of rows may match most of the
>> pages. It's great for finding strange behaviors / corner cases, but
>> probably not a sufficient reason to change the default.
> 
> Yes, I would like to see results from a data set where selectivity is
> more correlated to pages/heap fetches. But, I'm not sure I see how
> that is related to prefetching when eic = 1.
> 
>> I think it makes sense to issue a prefetch one page ahead, before
>> reading/processing the preceding one, and it's fairly conservative
>> setting, and I assume the default was chosen for a reason / after
>> discussion.
> 
> Yes, I suppose the overhead of an fadvise does not compare to the IO
> latency of synchronously reading that block. Actually, I bet the
> regression I saw by accidentally moving BitmapAdjustPrefetchIterator()
> after table_scan_bitmap_next_block() would be similar to the
> regression introduced by making eic = 1 not prefetch.
> 
> When you think about IO concurrency = 1, it doesn't imply prefetching
> to me. But, I think we want to do the right thing and have parity with
> master.
> 
>> My suggestion would be to keep the master behavior unless not practical,
>> and then maybe discuss changing the details later. The patch is already
>> complicated enough, better to leave that discussion for later.
> 
> Agreed. Speaking of which, we need to add back use of tablespace IO
> concurrency for the streaming read API (which is used by
> BitmapHeapScan in master).
> 
>>> With very low selectivity, you are less likely to get readahead
>>> (right?) and similarly less likely to be able to build up > 8kB IOs --
>>> which is one of the main value propositions of the streaming read
>>> code. I imagine that this larger read benefit is part of why the
>>> performance is better at higher selectivities with the patch. This
>>> might be a silly experiment, but we could try decreasing
>>> MAX_BUFFERS_PER_TRANSFER on the patched version and see if the
>>> performance gains go away.
>>
>> Sure, I can do that. Do you have any particular suggestion what value to
>> use for MAX_BUFFERS_PER_TRANSFER?
> 
> I think setting it to 1 would be the same as always master -- doing
> only 8kB reads. The only thing about that is that I imagine the other
> streaming read code has some overhead which might end up being a
> regression on balance even with the prefetching if we aren't actually
> using the ranges/vectored capabilities of the streaming read
> interface. Maybe if you just run it for one of the very obvious
> performance improvement cases? I can also try this locally.
> 

Here's some results from a build with

  #define MAX_BUFFERS_PER_TRANSFER 1

There are three columns:

- master
- patched (original patches, with MAX_BUFFERS_PER_TRANSFER=128kB)
- patched-single (MAX_BUFFERS_PER_TRANSFER=8kB)

The color scales are always branch compared to master.

I think the expectation was that setting the transfer to 1 would make it
closer to master, reducing some of the regressions. But in practice the
effect is the opposite.

- In "cached" runs, this eliminates the small improvements (light
green), but leaves the regressions behind.

- In "uncached" runs, this exacerbates the regressions, particularly for
low selectivities (small values of matches).


I don't have a good intuition on why this would be happening :-(

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

03 March, 03:15:28

On Sat, Mar 2, 2024 at 6:59 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
>
>
> On 3/1/24 17:51, Melanie Plageman wrote:
> > On Fri, Mar 1, 2024 at 9:05 AM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >>
> >> On 3/1/24 02:18, Melanie Plageman wrote:
> >>> On Thu, Feb 29, 2024 at 6:44 PM Tomas Vondra
> >>> <tomas.vondra@enterprisedb.com> wrote:
> >>>>
> >>>> On 2/29/24 23:44, Tomas Vondra wrote:
> >>>> 1) On master there's clear difference between eic=0 and eic=1 cases, but
> >>>> on the patched build there's literally no difference - for example the
> >>>> "uniform" distribution is clearly not great for prefetching, but eic=0
> >>>> regresses to eic=1 poor behavior).
> >>>
> >>> Yes, so eic=0 and eic=1 are identical with the streaming read API.
> >>> That is, eic 0 does not disable prefetching. Thomas is going to update
> >>> the streaming read API to avoid issuing an fadvise for the last block
> >>> in a range before issuing a read -- which would mean no prefetching
> >>> with eic 0 and eic 1. Not doing prefetching with eic 1 actually seems
> >>> like the right behavior -- which would be different than what master
> >>> is doing, right?
> >>
> >> I don't think we should stop doing prefetching for eic=1, or at least
> >> not based just on these charts. I suspect these "uniform" charts are not
> >> a great example for the prefetching, because it's about distribution of
> >> individual rows, and even a small fraction of rows may match most of the
> >> pages. It's great for finding strange behaviors / corner cases, but
> >> probably not a sufficient reason to change the default.
> >
> > Yes, I would like to see results from a data set where selectivity is
> > more correlated to pages/heap fetches. But, I'm not sure I see how
> > that is related to prefetching when eic = 1.
> >
> >> I think it makes sense to issue a prefetch one page ahead, before
> >> reading/processing the preceding one, and it's fairly conservative
> >> setting, and I assume the default was chosen for a reason / after
> >> discussion.
> >
> > Yes, I suppose the overhead of an fadvise does not compare to the IO
> > latency of synchronously reading that block. Actually, I bet the
> > regression I saw by accidentally moving BitmapAdjustPrefetchIterator()
> > after table_scan_bitmap_next_block() would be similar to the
> > regression introduced by making eic = 1 not prefetch.
> >
> > When you think about IO concurrency = 1, it doesn't imply prefetching
> > to me. But, I think we want to do the right thing and have parity with
> > master.
> >
> >> My suggestion would be to keep the master behavior unless not practical,
> >> and then maybe discuss changing the details later. The patch is already
> >> complicated enough, better to leave that discussion for later.
> >
> > Agreed. Speaking of which, we need to add back use of tablespace IO
> > concurrency for the streaming read API (which is used by
> > BitmapHeapScan in master).
> >
> >>> With very low selectivity, you are less likely to get readahead
> >>> (right?) and similarly less likely to be able to build up > 8kB IOs --
> >>> which is one of the main value propositions of the streaming read
> >>> code. I imagine that this larger read benefit is part of why the
> >>> performance is better at higher selectivities with the patch. This
> >>> might be a silly experiment, but we could try decreasing
> >>> MAX_BUFFERS_PER_TRANSFER on the patched version and see if the
> >>> performance gains go away.
> >>
> >> Sure, I can do that. Do you have any particular suggestion what value to
> >> use for MAX_BUFFERS_PER_TRANSFER?
> >
> > I think setting it to 1 would be the same as always master -- doing
> > only 8kB reads. The only thing about that is that I imagine the other
> > streaming read code has some overhead which might end up being a
> > regression on balance even with the prefetching if we aren't actually
> > using the ranges/vectored capabilities of the streaming read
> > interface. Maybe if you just run it for one of the very obvious
> > performance improvement cases? I can also try this locally.
> >
>
> Here's some results from a build with
>
>   #define MAX_BUFFERS_PER_TRANSFER 1
>
> There are three columns:
>
> - master
> - patched (original patches, with MAX_BUFFERS_PER_TRANSFER=128kB)
> - patched-single (MAX_BUFFERS_PER_TRANSFER=8kB)
>
> The color scales are always branch compared to master.
>
> I think the expectation was that setting the transfer to 1 would make it
> closer to master, reducing some of the regressions. But in practice the
> effect is the opposite.
>
> - In "cached" runs, this eliminates the small improvements (light
> green), but leaves the regressions behind.

For cached runs, I actually would expect that MAX_BUFFERS_PER_TRANSFER
would eliminate the regressions. Pinning more buffers will only hurt
us for cached workloads. This is evidence that we may need to control
the number of pinned buffers differently when there has been a run of
fully cached blocks.

> - In "uncached" runs, this exacerbates the regressions, particularly for
> low selectivities (small values of matches).

For the uncached runs, I actually expected it to eliminate the
performance gains that we saw with the patches applied. With
MAX_BUFFERS_PER_TRANSFER=1, we don't get the benefit of larger IOs and
fewer system calls but we still have the overhead of the streaming
read machinery. I was hoping to prove that the performance
improvements we saw with all the patches applied were due to
MAX_BUFFERS_PER_TRANSFER being > 1 causing fewer, bigger reads.

It did eliminate some performance gains, however, primarily for
cyclic-fuzz at lower selectivities. I am a little confused by this
part because with lower selectivities there are likely fewer
consecutive blocks that can be combined into one IO.

And, on average, we still see a lot of performance improvements that
were not eliminated by MAX_BUFFERS_PER_TRANSFER = 1. Hmm.

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

03 March, 17:36:23

On 3/3/24 00:39, Melanie Plageman wrote:
> On Sat, Mar 2, 2024 at 5:51 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 3/2/24 23:11, Melanie Plageman wrote:
>>> On Fri, Mar 1, 2024 at 2:31 PM Melanie Plageman
>>> <melanieplageman@gmail.com> wrote:
>>>>
>>>> ...
>>>>
>>>> Hold the phone on this one. I realized why I moved
>>>> BitmapAdjustPrefetchIterator after table_scan_bitmap_next_block() in
>>>> the first place -- master calls BitmapAdjustPrefetchIterator after the
>>>> tbm_iterate() for the current block -- otherwise with eic = 1, it
>>>> considers the prefetch iterator behind the current block iterator. I'm
>>>> going to go through and figure out what order this must be done in and
>>>> fix it.
>>>
>>> So, I investigated this further, and, as far as I can tell, for
>>> parallel bitmapheapscan the timing around when workers decrement
>>> prefetch_pages causes the performance differences with patch 0010
>>> applied. It makes very little sense to me, but some of the queries I
>>> borrowed from your regression examples are up to 30% slower when this
>>> code from BitmapAdjustPrefetchIterator() is after
>>> table_scan_bitmap_next_block() instead of before it.
>>>
>>>         SpinLockAcquire(&pstate->mutex);
>>>         if (pstate->prefetch_pages > 0)
>>>             pstate->prefetch_pages--;
>>>         SpinLockRelease(&pstate->mutex);
>>>
>>> I did some stracing and did see much more time spent in futex/wait
>>> with this code after the call to table_scan_bitmap_next_block() vs
>>> before it. (table_scan_bitmap_next_block()) calls ReadBuffer()).
>>>
>>> In my branch, I've now moved only the parallel prefetch_pages-- code
>>> to before table_scan_bitmap_next_block().
>>> https://github.com/melanieplageman/postgres/tree/bhs_pgsr
>>> I'd be interested to know if you see the regressions go away with 0010
>>> applied (commit message "Make table_scan_bitmap_next_block() async
>>> friendly" and sha bfdcbfee7be8e2c461).
>>>
>>
>> I'll give this a try once the runs with MAX_BUFFERS_PER_TRANSFER=1
>> complete. But it seems really bizarre that simply moving this code a
>> little bit would cause such a regression ...
> 
> Yes, it is bizarre. It also might not be a reproducible performance
> difference on the cases besides the one I was testing (cyclic dataset,
> uncached, eic=8, matches 16+, distinct=100, rows=100000000, 4 parallel
> workers). But even if it only affects that one case, it still had a
> major, reproducible performance impact to move those 5 lines before
> and after table_scan_bitmap_next_block().
> 
> The same number of reads and fadvises are being issued overall.
> However, I did notice that the pread calls are skewed when the those
> lines of code are after table_scan_bitmap_next_block()  -- fewer of
> the workers are doing more of the reads. Perhaps this explains what is
> taking longer. Why those workers would end up doing more of the reads,
> I don't quite know.
> 
> - Melanie


I do have some numbers with e44505ce179e442bd50664c85a31a1805e13514a,
and I don't see any such effect - it performs pretty much exactly like
the v6 patches.

I used a slightly different visualization, plotting the timings on a
scatter plot, so values on diagonal mean "same performance" while values
above/below mean speedup/slowdown.

This is a bit more compact than the tables with color scales, and it
makes it harder (impossible) to see patterns (e.g. changes depending on
eic). But for evaluating if there's a shift overall it still works, and
it also shows clusters. So more a complementary & simpler visualization.

There are three charts

1) master-patched.png - master vs. v6 patches
2) master-locks.png - master vs. e44505ce
3) patched-locks.png - v6 patches vs. e44505ce

There's virtually no difference between (1) and (2) - same pattern of
regressions and speedups, almost as a copy. That's confirmed by (3)
where pretty much all values are exactly on the diagonal, with only a
couple outliers.

I'm not sure why you see a 30% difference with the change. I wonder if
that might be due to some issue in the environment? Are you running in a
VM, or something like that?

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Heikki Linnakangas

Date:

13 March, 16:34:15

(Adding Dilip, the original author of the parallel bitmap heap scan 
patch all those years ago, in case you remember anything about the 
snapshot stuff below.)

On 27/02/2024 16:22, Melanie Plageman wrote:
> On Mon, Feb 26, 2024 at 08:50:28PM -0500, Melanie Plageman wrote:
>> On Fri, Feb 16, 2024 at 12:35:59PM -0500, Melanie Plageman wrote:
>>> In the attached v3, I've reordered the commits, updated some errant
>>> comments, and improved the commit messages.
>>>
>>> I've also made some updates to the TIDBitmap API that seem like a
>>> clarity improvement to the API in general. These also reduce the diff
>>> for GIN when separating the TBMIterateResult from the
>>> TBM[Shared]Iterator. And these TIDBitmap API changes are now all in
>>> their own commits (previously those were in the same commit as adding
>>> the BitmapHeapScan streaming read user).
>>>
>>> The three outstanding issues I see in the patch set are:
>>> 1) the lossy and exact page counters issue described in my previous
>>> email
>>
>> I've resolved this. I added a new patch to the set which starts counting
>> even pages with no visible tuples toward lossy and exact pages. After an
>> off-list conversation with Andres, it seems that this omission in master
>> may not have been intentional.
>>
>> Once we have only two types of pages to differentiate between (lossy and
>> exact [no longer have to care about "has no visible tuples"]), it is
>> easy enough to pass a "lossy" boolean paramater to
>> table_scan_bitmap_next_block(). I've done this in the attached v4.
> 
> Thomas posted a new version of the Streaming Read API [1], so here is a
> rebased v5. This should make it easier to review as it can be applied on
> top of master.

Lots of discussion happening on the performance results but it seems 
that there is no performance impact with the preliminary patches up to 
v5-0013-Streaming-Read-API.patch. I'm focusing purely on those 
preliminary patches now, because I think they're worthwhile cleanups 
independent of the streaming read API.

Andres already commented on the snapshot stuff on an earlier patch 
version, and that's much nicer with this version. However, I don't 
understand why a parallel bitmap heap scan needs to do anything at all 
with the snapshot, even before these patches. The parallel worker 
infrastructure already passes the active snapshot from the leader to the 
parallel worker. Why does bitmap heap scan code need to do that too?

I disabled that with:

> --- a/src/backend/executor/nodeBitmapHeapscan.c
> +++ b/src/backend/executor/nodeBitmapHeapscan.c
> @@ -874,7 +874,9 @@ ExecBitmapHeapInitializeWorker(BitmapHeapScanState *node,
>      pstate = shm_toc_lookup(pwcxt->toc, node->ss.ps.plan->plan_node_id, false);
>      node->pstate = pstate;
>  
> +#if 0
>      node->worker_snapshot = RestoreSnapshot(pstate->phs_snapshot_data);
>      Assert(IsMVCCSnapshot(node->worker_snapshot));
>      RegisterSnapshot(node->worker_snapshot);
> +#endif
>  }

and ran "make check-world". All the tests passed. To be even more sure, 
I added some code there to assert that the serialized version of 
node->ss.ps.state->es_snapshot is equal to pstate->phs_snapshot_data, 
and all the tests passed with that too.

I propose that we just remove the code in BitmapHeapScan to serialize 
the snapshot, per attached patch.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Attachment

0001-Remove-redundant-snapshot-copying-from-parallel-lead.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Dilip Kumar

Date:

13 March, 18:39:04

On Wed, Mar 13, 2024 at 7:04 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> (Adding Dilip, the original author of the parallel bitmap heap scan
> patch all those years ago, in case you remember anything about the
> snapshot stuff below.)
>
> On 27/02/2024 16:22, Melanie Plageman wrote:

> Andres already commented on the snapshot stuff on an earlier patch
> version, and that's much nicer with this version. However, I don't
> understand why a parallel bitmap heap scan needs to do anything at all
> with the snapshot, even before these patches. The parallel worker
> infrastructure already passes the active snapshot from the leader to the
> parallel worker. Why does bitmap heap scan code need to do that too?

Yeah thinking on this now it seems you are right that the parallel
infrastructure is already passing the active snapshot so why do we
need it again.  Then I checked other low scan nodes like indexscan and
seqscan and it seems we are doing the same things there as well.
Check for SerializeSnapshot() in table_parallelscan_initialize() and
index_parallelscan_initialize() which are being called from
ExecSeqScanInitializeDSM() and ExecIndexScanInitializeDSM()
respectively.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Robert Haas

Date:

13 March, 18:55:33

On Wed, Mar 13, 2024 at 11:39 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > Andres already commented on the snapshot stuff on an earlier patch
> > version, and that's much nicer with this version. However, I don't
> > understand why a parallel bitmap heap scan needs to do anything at all
> > with the snapshot, even before these patches. The parallel worker
> > infrastructure already passes the active snapshot from the leader to the
> > parallel worker. Why does bitmap heap scan code need to do that too?
>
> Yeah thinking on this now it seems you are right that the parallel
> infrastructure is already passing the active snapshot so why do we
> need it again.  Then I checked other low scan nodes like indexscan and
> seqscan and it seems we are doing the same things there as well.
> Check for SerializeSnapshot() in table_parallelscan_initialize() and
> index_parallelscan_initialize() which are being called from
> ExecSeqScanInitializeDSM() and ExecIndexScanInitializeDSM()
> respectively.

I remember thinking about this when I was writing very early parallel
query code. It seemed to me that there must be some reason why the
EState has a snapshot, as opposed to just using the active snapshot,
and so I took care to propagate that snapshot, which is used for the
leader's scans, to the worker scans also. Now, if the EState doesn't
need to contain a snapshot, then all of that mechanism is unnecessary,
but I don't see how it can be right for the leader to do
table_beginscan() using estate->es_snapshot and the worker to use the
active snapshot.

--
Robert Haas
EDB: http://www.enterprisedb.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

13 March, 21:14:36

On 3/13/24 14:34, Heikki Linnakangas wrote:
> ...
> 
> Lots of discussion happening on the performance results but it seems
> that there is no performance impact with the preliminary patches up to
> v5-0013-Streaming-Read-API.patch. I'm focusing purely on those
> preliminary patches now, because I think they're worthwhile cleanups
> independent of the streaming read API.
> 

Not quite true - the comparison I shared on 29/2 [1] shows a serious
regression caused by the 0010 patch. We've been investigating this with
Melanie off list, but we don't have any clear findings yet (except that
it's clearly due to moving BitmapAdjustPrefetchIterator() a bit down.

But if we revert this (and move the BitmapAdjustPrefetchIterator back),
the regression should disappear, and we can merge these preparatory
patches. We'll have to deal with the regression (or something very
similar) when merging the remaining patches.

regards

[1]
https://www.postgresql.org/message-id/91090d58-7d3f-4447-9425-f24ba66e292a%40enterprisedb.com
-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Thomas Munro

Date:

14 March, 01:38:38

On Sun, Mar 3, 2024 at 11:41 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
> On 3/2/24 23:28, Melanie Plageman wrote:
> > On Sat, Mar 2, 2024 at 10:05 AM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >> With the current "master" code, eic=1 means we'll issue a prefetch for B
> >> and then read+process A. And then issue prefetch for C and read+process
> >> B, and so on. It's always one page ahead.
> >
> > Yes, that is what I mean for eic = 1

I spent quite a few days thinking about the meaning of eic=0 and eic=1
for streaming_read.c v7[1], to make it agree with the above and with
master.  Here's why I was confused:

Both eic=0 and eic=1 are expected to generate at most 1 physical I/O
at a time, or I/O queue depth 1 if you want to put it that way.  But
this isn't just about concurrency of I/O, it's also about computation.
Duh.

eic=0 means that the I/O is not concurrent with executor computation.
So, to annotate an excerpt from [1]'s random.txt, we have:

effective_io_concurrency = 0, range size = 1
unpatched                              patched
==============================================================================
pread(43,...,8192,0x58000) = 8192      pread(82,...,8192,0x58000) = 8192
             *** executor now has page at 0x58000 to work on ***
pread(43,...,8192,0xb0000) = 8192      pread(82,...,8192,0xb0000) = 8192
             *** executor now has page at 0xb0000 to work on ***

eic=1 means that a single I/O is started and then control is returned
to the executor code to do useful work concurrently with the
background read that we assume is happening:

effective_io_concurrency = 1, range size = 1
unpatched                              patched
==============================================================================
pread(43,...,8192,0x58000) = 8192      pread(82,...,8192,0x58000) = 8192
posix_fadvise(43,0xb0000,0x2000,...)   posix_fadvise(82,0xb0000,0x2000,...)
             *** executor now has page at 0x58000 to work on ***
pread(43,...,8192,0xb0000) = 8192      pread(82,...,8192,0xb0000) = 8192
posix_fadvise(43,0x108000,0x2000,...)  posix_fadvise(82,0x108000,0x2000,...)
             *** executor now has page at 0xb0000 to work on ***
pread(43,...,8192,0x108000) = 8192     pread(82,...,8192,0x108000) = 8192
posix_fadvise(43,0x160000,0x2000,...)  posix_fadvise(82,0x160000,0x2000,...)

In other words, 'concurrency' doesn't mean 'number of I/Os running
concurrently with each other', it means 'number of I/Os running
concurrently with computation', and when you put it that way, 0 and 1
are different.

Note that the first read is a bit special: by the time the consumer is
ready to pull a buffer out of the stream when we don't have a buffer
ready yet, it is too late to issue useful advice, so we don't bother.
FWIW I think even in the AIO future we would have a synchronous read
in that specific place, at least when using io_method=worker, because
it would be stupid to ask another process to read a block for us that
we want right now and then wait for it wake us up when it's done.

Note that even when we aren't issuing any advice because eic=0 or
because we detected sequential access and we believe the kernel can do
a better job than us, we still 'look ahead' (= call the callback to
see which block numbers are coming down the pipe), but only as far as
we need to coalesce neighbouring blocks.  (I deliberately avoid using
the word "prefetch" except in very general discussions because it
means different things to different layers of the code, hence talk of
"look ahead" and "advice".)  That's how we get this change:

effective_io_concurrency = 0, range size = 4
unpatched                              patched
==============================================================================
pread(43,...,8192,0x58000) = 8192      pread(82,...,8192,0x58000) = 8192
pread(43,...,8192,0x5a000) = 8192      preadv(82,...,2,0x5a000) = 16384
pread(43,...,8192,0x5c000) = 8192      pread(82,...,8192,0x5e000) = 8192
pread(43,...,8192,0x5e000) = 8192      preadv(82,...,4,0xb0000) = 32768
pread(43,...,8192,0xb0000) = 8192      preadv(82,...,4,0x108000) = 32768
pread(43,...,8192,0xb2000) = 8192      preadv(82,...,4,0x160000) = 32768

And then once we introduce eic > 0 to the picture with neighbouring
blocks that can be coalesced, "patched" starts to diverge even more
from "unpatched" because it tracks the number of wide I/Os in
progress, not the number of single blocks.

[1] https://www.postgresql.org/message-id/CA+hUKGLJi+c5jB3j6UvkgMYHky-qu+LPCsiNahUGSa5Z4DvyVA@mail.gmail.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Dilip Kumar

Date:

14 March, 07:54:14

On Wed, Mar 13, 2024 at 9:25 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Wed, Mar 13, 2024 at 11:39 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > > Andres already commented on the snapshot stuff on an earlier patch
> > > version, and that's much nicer with this version. However, I don't
> > > understand why a parallel bitmap heap scan needs to do anything at all
> > > with the snapshot, even before these patches. The parallel worker
> > > infrastructure already passes the active snapshot from the leader to the
> > > parallel worker. Why does bitmap heap scan code need to do that too?
> >
> > Yeah thinking on this now it seems you are right that the parallel
> > infrastructure is already passing the active snapshot so why do we
> > need it again.  Then I checked other low scan nodes like indexscan and
> > seqscan and it seems we are doing the same things there as well.
> > Check for SerializeSnapshot() in table_parallelscan_initialize() and
> > index_parallelscan_initialize() which are being called from
> > ExecSeqScanInitializeDSM() and ExecIndexScanInitializeDSM()
> > respectively.
>
> I remember thinking about this when I was writing very early parallel
> query code. It seemed to me that there must be some reason why the
> EState has a snapshot, as opposed to just using the active snapshot,
> and so I took care to propagate that snapshot, which is used for the
> leader's scans, to the worker scans also. Now, if the EState doesn't
> need to contain a snapshot, then all of that mechanism is unnecessary,
> but I don't see how it can be right for the leader to do
> table_beginscan() using estate->es_snapshot and the worker to use the
> active snapshot.

Yeah, that's a very valid point. So I think now Heikki/Melanie might
have got an answer to their question, about the thought process behind
serializing the snapshot for each scan node.  And the same thing is
followed for BitmapHeapNode as well.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Heikki Linnakangas

Date:

14 March, 13:37:02

On 14/03/2024 06:54, Dilip Kumar wrote:
> On Wed, Mar 13, 2024 at 9:25 PM Robert Haas <robertmhaas@gmail.com> wrote:
>>
>> On Wed, Mar 13, 2024 at 11:39 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>>>> Andres already commented on the snapshot stuff on an earlier patch
>>>> version, and that's much nicer with this version. However, I don't
>>>> understand why a parallel bitmap heap scan needs to do anything at all
>>>> with the snapshot, even before these patches. The parallel worker
>>>> infrastructure already passes the active snapshot from the leader to the
>>>> parallel worker. Why does bitmap heap scan code need to do that too?
>>>
>>> Yeah thinking on this now it seems you are right that the parallel
>>> infrastructure is already passing the active snapshot so why do we
>>> need it again.  Then I checked other low scan nodes like indexscan and
>>> seqscan and it seems we are doing the same things there as well.
>>> Check for SerializeSnapshot() in table_parallelscan_initialize() and
>>> index_parallelscan_initialize() which are being called from
>>> ExecSeqScanInitializeDSM() and ExecIndexScanInitializeDSM()
>>> respectively.
>>
>> I remember thinking about this when I was writing very early parallel
>> query code. It seemed to me that there must be some reason why the
>> EState has a snapshot, as opposed to just using the active snapshot,
>> and so I took care to propagate that snapshot, which is used for the
>> leader's scans, to the worker scans also. Now, if the EState doesn't
>> need to contain a snapshot, then all of that mechanism is unnecessary,
>> but I don't see how it can be right for the leader to do
>> table_beginscan() using estate->es_snapshot and the worker to use the
>> active snapshot.
> 
> Yeah, that's a very valid point. So I think now Heikki/Melanie might
> have got an answer to their question, about the thought process behind
> serializing the snapshot for each scan node.  And the same thing is
> followed for BitmapHeapNode as well.

I see. Thanks, understanding the thought process helps.

So when a parallel table or index scan runs in the executor as part of a 
query, we could just use the active snapshot. But there are some other 
callers of parallel table scans that don't use the executor, namely 
parallel index builds. For those it makes sense to pass the snapshot for 
the scan independent of the active snapshot.

A parallel bitmap heap scan isn't really a parallel scan as far as the 
table AM is concerned, though. It's more like an independent bitmap heap 
scan in each worker process, nodeBitmapHeapscan.c does all the 
coordination of which blocks to scan. So I think that 
table_parallelscan_initialize() was the wrong role model, and we should 
still remove the snapshot serialization code from nodeBitmapHeapscan.c.

Digging deeper into the question of whether es_snapshot == 
GetActiveSnapshot() is a valid assumption:

<deep dive>

es_snapshot is copied from the QueryDesc in standard_ExecutorStart(). 
Looking at the callers of ExecutorStart(), they all get the QueryDesc by 
calling CreateQueryDesc() with GetActiveSnapshot(). And I don't see any 
callers changing the active snapshot between the ExecutorStart() and 
ExecutorRun() calls either. In pquery.c, we explicitly 
PushActiveSnapshot(queryDesc->snapshot) before calling ExecutorRun(). So 
no live bug here AFAICS, es_snapshot == GetActiveSnapshot() holds.

_SPI_execute_plan() has code to deal with the possibility that the 
active snapshot is not set. That seems fishy; do we really support SPI 
without any snapshot? I'm inclined to turn that into an error. I ran the 
regression tests with an "Assert(ActiveSnapshotSet())" there, and 
everything worked.

If es_snapshot was different from the active snapshot, things would get 
weird, even without parallel query. The scans would use es_snapshot for 
the visibility checks, but any functions you execute in quals would use 
the active snapshot.

We could double down on that assumption, and remove es_snapshot 
altogether and use GetActiveSnapshot() instead. And perhaps add 
"PushActiveSnapshot(queryDesc->snapshot)" to ExecutorRun().

</deep dive>

In summary, this es_snapshot stuff is a bit confusing and could use some 
cleanup. But for now, I'd like to just add some assertions and a 
comments about this, and remove the snapshot serialization from bitmap 
heap scan node, to make it consistent with other non-parallel scan nodes 
(it's not really a parallel scan as far as the table AM is concerned). 
See attached patch, which is the same as previous patch with some extra 
assertions.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Attachment

v2-0001-Remove-redundant-snapshot-copying-from-parallel-l.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Dilip Kumar

Date:

14 March, 13:55:34

On Thu, Mar 14, 2024 at 4:07 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> > Yeah, that's a very valid point. So I think now Heikki/Melanie might
> > have got an answer to their question, about the thought process behind
> > serializing the snapshot for each scan node.  And the same thing is
> > followed for BitmapHeapNode as well.
>
> I see. Thanks, understanding the thought process helps.
>
> So when a parallel table or index scan runs in the executor as part of a
> query, we could just use the active snapshot. But there are some other
> callers of parallel table scans that don't use the executor, namely
> parallel index builds. For those it makes sense to pass the snapshot for
> the scan independent of the active snapshot.

Right

> A parallel bitmap heap scan isn't really a parallel scan as far as the
> table AM is concerned, though. It's more like an independent bitmap heap
> scan in each worker process, nodeBitmapHeapscan.c does all the
> coordination of which blocks to scan. So I think that
> table_parallelscan_initialize() was the wrong role model, and we should
> still remove the snapshot serialization code from nodeBitmapHeapscan.c.

I think that seems right.

> Digging deeper into the question of whether es_snapshot ==
> GetActiveSnapshot() is a valid assumption:
>
> <deep dive>
>
> es_snapshot is copied from the QueryDesc in standard_ExecutorStart().
> Looking at the callers of ExecutorStart(), they all get the QueryDesc by
> calling CreateQueryDesc() with GetActiveSnapshot(). And I don't see any
> callers changing the active snapshot between the ExecutorStart() and
> ExecutorRun() calls either. In pquery.c, we explicitly
> PushActiveSnapshot(queryDesc->snapshot) before calling ExecutorRun(). So
> no live bug here AFAICS, es_snapshot == GetActiveSnapshot() holds.
>
> _SPI_execute_plan() has code to deal with the possibility that the
> active snapshot is not set. That seems fishy; do we really support SPI
> without any snapshot? I'm inclined to turn that into an error. I ran the
> regression tests with an "Assert(ActiveSnapshotSet())" there, and
> everything worked.

IMHO, we can call SPI_Connect() and SPI_Execute() from any C
extension, so I don't think there we can guarantee that the snapshot
must be set, do we?

> If es_snapshot was different from the active snapshot, things would get
> weird, even without parallel query. The scans would use es_snapshot for
> the visibility checks, but any functions you execute in quals would use
> the active snapshot.
>
> We could double down on that assumption, and remove es_snapshot
> altogether and use GetActiveSnapshot() instead. And perhaps add
> "PushActiveSnapshot(queryDesc->snapshot)" to ExecutorRun().
>
> </deep dive>
>
> In summary, this es_snapshot stuff is a bit confusing and could use some
> cleanup. But for now, I'd like to just add some assertions and a
> comments about this, and remove the snapshot serialization from bitmap
> heap scan node, to make it consistent with other non-parallel scan nodes
> (it's not really a parallel scan as far as the table AM is concerned).
> See attached patch, which is the same as previous patch with some extra
> assertions.

Maybe for now we can just handle this specific case to remove the
snapshot serializing for the BitmapHeapScan as you are doing in the
patch.  After looking into the code your theory seems correct that we
are just copying the ActiveSnapshot while building the query
descriptor and from there we are copying into the Estate so logically
there should not be any reason for these two to be different.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Robert Haas

Date:

14 March, 15:34:37

On Thu, Mar 14, 2024 at 6:37 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> If es_snapshot was different from the active snapshot, things would get
> weird, even without parallel query. The scans would use es_snapshot for
> the visibility checks, but any functions you execute in quals would use
> the active snapshot.

Hmm, that's an interesting point.

The case where the query is suspended and resumed - i.e. cursors are
used - probably needs more analysis. In that case, perhaps there's
more room for the snapshots to diverge.

--
Robert Haas
EDB: http://www.enterprisedb.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Heikki Linnakangas

Date:

14 March, 16:00:49

On 14/03/2024 14:34, Robert Haas wrote:
> On Thu, Mar 14, 2024 at 6:37 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> If es_snapshot was different from the active snapshot, things would get
>> weird, even without parallel query. The scans would use es_snapshot for
>> the visibility checks, but any functions you execute in quals would use
>> the active snapshot.
> 
> Hmm, that's an interesting point.
> 
> The case where the query is suspended and resumed - i.e. cursors are
> used - probably needs more analysis. In that case, perhaps there's
> more room for the snapshots to diverge.

The portal code is pretty explicit about it, the ExecutorRun() call in 
PortalRunSelect() looks like this:

             PushActiveSnapshot(queryDesc->snapshot);
             ExecutorRun(queryDesc, direction, (uint64) count,
                         portal->run_once);
             nprocessed = queryDesc->estate->es_processed;
             PopActiveSnapshot();

I looked at all the callers of ExecutorRun(), and they all have the 
active snapshot equal to queryDesc->snapshot, either because they called 
CreateQueryDesc() with the active snapshot before ExecutorRun(), or they 
set the active snapshot like above.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Robert Haas

Date:

14 March, 16:20:56

On Thu, Mar 14, 2024 at 9:00 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> The portal code is pretty explicit about it, the ExecutorRun() call in
> PortalRunSelect() looks like this:
>
>              PushActiveSnapshot(queryDesc->snapshot);
>              ExecutorRun(queryDesc, direction, (uint64) count,
>                          portal->run_once);
>              nprocessed = queryDesc->estate->es_processed;
>              PopActiveSnapshot();
>
> I looked at all the callers of ExecutorRun(), and they all have the
> active snapshot equal to queryDesc->snapshot, either because they called
> CreateQueryDesc() with the active snapshot before ExecutorRun(), or they
> set the active snapshot like above.

Well, maybe there's a bunch of code cleanup possible, then.

--
Robert Haas
EDB: http://www.enterprisedb.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Heikki Linnakangas

Date:

14 March, 16:32:04

On 14/03/2024 12:55, Dilip Kumar wrote:
> On Thu, Mar 14, 2024 at 4:07 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> _SPI_execute_plan() has code to deal with the possibility that the
>> active snapshot is not set. That seems fishy; do we really support SPI
>> without any snapshot? I'm inclined to turn that into an error. I ran the
>> regression tests with an "Assert(ActiveSnapshotSet())" there, and
>> everything worked.
> 
> IMHO, we can call SPI_Connect() and SPI_Execute() from any C
> extension, so I don't think there we can guarantee that the snapshot
> must be set, do we?

I suppose, although the things you could do without a snapshot would be 
pretty limited. The query couldn't access any tables. Could it even look 
up functions in the parser? Not sure.

> Maybe for now we can just handle this specific case to remove the
> snapshot serializing for the BitmapHeapScan as you are doing in the
> patch.  After looking into the code your theory seems correct that we
> are just copying the ActiveSnapshot while building the query
> descriptor and from there we are copying into the Estate so logically
> there should not be any reason for these two to be different.

Ok, committed that for now. Thanks for looking!

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

14 March, 17:17:59


On 3/13/24 23:38, Thomas Munro wrote:
> On Sun, Mar 3, 2024 at 11:41 AM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>> On 3/2/24 23:28, Melanie Plageman wrote:
>>> On Sat, Mar 2, 2024 at 10:05 AM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>> With the current "master" code, eic=1 means we'll issue a prefetch for B
>>>> and then read+process A. And then issue prefetch for C and read+process
>>>> B, and so on. It's always one page ahead.
>>>
>>> Yes, that is what I mean for eic = 1
> 
> I spent quite a few days thinking about the meaning of eic=0 and eic=1
> for streaming_read.c v7[1], to make it agree with the above and with
> master.  Here's why I was confused:
> 
> Both eic=0 and eic=1 are expected to generate at most 1 physical I/O
> at a time, or I/O queue depth 1 if you want to put it that way.  But
> this isn't just about concurrency of I/O, it's also about computation.
> Duh.
> 
> eic=0 means that the I/O is not concurrent with executor computation.
> So, to annotate an excerpt from [1]'s random.txt, we have:
> 
> effective_io_concurrency = 0, range size = 1
> unpatched                              patched
> ==============================================================================
> pread(43,...,8192,0x58000) = 8192      pread(82,...,8192,0x58000) = 8192
>              *** executor now has page at 0x58000 to work on ***
> pread(43,...,8192,0xb0000) = 8192      pread(82,...,8192,0xb0000) = 8192
>              *** executor now has page at 0xb0000 to work on ***
> 
> eic=1 means that a single I/O is started and then control is returned
> to the executor code to do useful work concurrently with the
> background read that we assume is happening:
> 
> effective_io_concurrency = 1, range size = 1
> unpatched                              patched
> ==============================================================================
> pread(43,...,8192,0x58000) = 8192      pread(82,...,8192,0x58000) = 8192
> posix_fadvise(43,0xb0000,0x2000,...)   posix_fadvise(82,0xb0000,0x2000,...)
>              *** executor now has page at 0x58000 to work on ***
> pread(43,...,8192,0xb0000) = 8192      pread(82,...,8192,0xb0000) = 8192
> posix_fadvise(43,0x108000,0x2000,...)  posix_fadvise(82,0x108000,0x2000,...)
>              *** executor now has page at 0xb0000 to work on ***
> pread(43,...,8192,0x108000) = 8192     pread(82,...,8192,0x108000) = 8192
> posix_fadvise(43,0x160000,0x2000,...)  posix_fadvise(82,0x160000,0x2000,...)
> 
> In other words, 'concurrency' doesn't mean 'number of I/Os running
> concurrently with each other', it means 'number of I/Os running
> concurrently with computation', and when you put it that way, 0 and 1
> are different.
> 

Interesting. For some reason I thought with eic=1 we'd issue the fadvise
for page #2 before pread of page #1, so that there'd be 2 IO requests in
flight at the same time for a bit of time ... it'd give the fadvise more
time to actually get the data into page cache.

> Note that the first read is a bit special: by the time the consumer is
> ready to pull a buffer out of the stream when we don't have a buffer
> ready yet, it is too late to issue useful advice, so we don't bother.
> FWIW I think even in the AIO future we would have a synchronous read
> in that specific place, at least when using io_method=worker, because
> it would be stupid to ask another process to read a block for us that
> we want right now and then wait for it wake us up when it's done.
> 
> Note that even when we aren't issuing any advice because eic=0 or
> because we detected sequential access and we believe the kernel can do
> a better job than us, we still 'look ahead' (= call the callback to
> see which block numbers are coming down the pipe), but only as far as
> we need to coalesce neighbouring blocks.  (I deliberately avoid using
> the word "prefetch" except in very general discussions because it
> means different things to different layers of the code, hence talk of
> "look ahead" and "advice".)  That's how we get this change:
> 
> effective_io_concurrency = 0, range size = 4
> unpatched                              patched
> ==============================================================================
> pread(43,...,8192,0x58000) = 8192      pread(82,...,8192,0x58000) = 8192
> pread(43,...,8192,0x5a000) = 8192      preadv(82,...,2,0x5a000) = 16384
> pread(43,...,8192,0x5c000) = 8192      pread(82,...,8192,0x5e000) = 8192
> pread(43,...,8192,0x5e000) = 8192      preadv(82,...,4,0xb0000) = 32768
> pread(43,...,8192,0xb0000) = 8192      preadv(82,...,4,0x108000) = 32768
> pread(43,...,8192,0xb2000) = 8192      preadv(82,...,4,0x160000) = 32768
> 
> And then once we introduce eic > 0 to the picture with neighbouring
> blocks that can be coalesced, "patched" starts to diverge even more
> from "unpatched" because it tracks the number of wide I/Os in
> progress, not the number of single blocks.
> 

So, IIUC this means (1) the patched code is more aggressive wrt
prefetching (because we prefetch more data overall, because master would
prefetch N pages and patched prefetches N ranges, each of which may be
multiple pages. And (2) it's not easy to quantify how much more
aggressive it is, because it depends on how we happen to coalesce the
pages into ranges.

Do I understand this correctly?


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

14 March, 21:16:25

On Thu, Mar 14, 2024 at 03:32:04PM +0200, Heikki Linnakangas wrote:
> On 14/03/2024 12:55, Dilip Kumar wrote:
> > On Thu, Mar 14, 2024 at 4:07 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> > > _SPI_execute_plan() has code to deal with the possibility that the
> > > active snapshot is not set. That seems fishy; do we really support SPI
> > > without any snapshot? I'm inclined to turn that into an error. I ran the
> > > regression tests with an "Assert(ActiveSnapshotSet())" there, and
> > > everything worked.
> > 
> > IMHO, we can call SPI_Connect() and SPI_Execute() from any C
> > extension, so I don't think there we can guarantee that the snapshot
> > must be set, do we?
> 
> I suppose, although the things you could do without a snapshot would be
> pretty limited. The query couldn't access any tables. Could it even look up
> functions in the parser? Not sure.
> 
> > Maybe for now we can just handle this specific case to remove the
> > snapshot serializing for the BitmapHeapScan as you are doing in the
> > patch.  After looking into the code your theory seems correct that we
> > are just copying the ActiveSnapshot while building the query
> > descriptor and from there we are copying into the Estate so logically
> > there should not be any reason for these two to be different.
> 
> Ok, committed that for now. Thanks for looking!

Attached v6 is rebased over your new commit. It also has the "fix" in
0010 which moves BitmapAdjustPrefetchIterator() back above
table_scan_bitmap_next_block(). I've also updated the Streaming Read API
commit (0013) to Thomas' v7 version from [1]. This has the update that
we theorize should address some of the regressions in the bitmapheapscan
streaming read user in 0014.

- Melanie

[1] https://www.postgresql.org/message-id/CA%2BhUKGLJi%2Bc5jB3j6UvkgMYHky-qu%2BLPCsiNahUGSa5Z4DvyVA%40mail.gmail.com

On 3/14/24 22:39, Melanie Plageman wrote:
> On Thu, Mar 14, 2024 at 5:26 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 3/14/24 19:16, Melanie Plageman wrote:
>>> On Thu, Mar 14, 2024 at 03:32:04PM +0200, Heikki Linnakangas wrote:
>>>> ...
>>>>
>>>> Ok, committed that for now. Thanks for looking!
>>>
>>> Attached v6 is rebased over your new commit. It also has the "fix" in
>>> 0010 which moves BitmapAdjustPrefetchIterator() back above
>>> table_scan_bitmap_next_block(). I've also updated the Streaming Read API
>>> commit (0013) to Thomas' v7 version from [1]. This has the update that
>>> we theorize should address some of the regressions in the bitmapheapscan
>>> streaming read user in 0014.
>>>
>>
>> Should I rerun the benchmarks with these new patches, to see if it
>> really helps with the regressions?
> 
> That would be awesome!
> 

OK, here's a couple charts comparing the effect of v6 patches to master.
These are from 1M and 10M data sets, same as the runs presented earlier
in this thread (the 10M is still running, but should be good enough for
this kind of visual comparison).

I have results for individual patches, but 0001-0013 behave virtually
the same, so the charts show only 0012 and 0014 (vs master).

Instead of a table with color scale (used before), I used simple scatter
plots as a more compact / concise visualization. It's impossible to
identify patterns (e.g. serial vs. parallel runs), but for the purpose
of this comparison that does not matter.

And then I'll use a chart plotting "relative" time compared to master (I
find it easier to judge the relative difference than with scatter plot).

1) absolute-all - all runs (scatter plot)

2) absolute-optimal - runs where the planner would pick bitmapscan

3) relative-all - all runs (duration relative to master)

4) relative-optimal - relative, runs where bitmapscan would be picked

The 0012 results are a pretty clear sign the "refactoring patches"
behave exactly the same as master. There are a couple outliers (in
either direction), but I'd attribute those to random noise and too few
runs to smooth it out for a particular combination (especially for 10M).

What is even more obvious is that 0014 behaves *VERY* differently. I'm
not sure if this is a good thing or a problem is debatable/unclear. I'm
sure we don't want to cause regressions, but perhaps those are due to
the prefetch issue discussed elsewhere in this thread (identified by
Andres and Melanie). There are also many cases that got much faster, but
the question is whether this is due to better efficiency or maybe the
new code being more aggressive in some way (not sure).

It's however interesting the differences are way more significant (both
in terms of frequency and scale) on the older machine with SATA SSDs.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Mon, Mar 18, 2024 at 02:10:28PM +0200, Heikki Linnakangas wrote:
> On 14/02/2024 21:42, Andres Freund wrote:
> > On 2024-02-13 18:11:25 -0500, Melanie Plageman wrote:
> > > patch 0004 is, I think, a bug fix. see [2].
> > 
> > I'd not quite call it a bugfix, it's not like it leads to wrong
> > behaviour. Seems more like an optimization. But whatever :)
> 
> It sure looks like bug to me, albeit a very minor one. Certainly not an
> optimization, it doesn't affect performance in any way, only what EXPLAIN
> reports. So committed and backported that to all supported branches.

I've attached v7 rebased over this commit.

- Melanie

On 3/18/24 16:55, Tomas Vondra wrote:
>
> ...
> 
> OK, I've restarted the tests for only 0012 and 0014 patches, and I'll
> wait for these to complete - I don't want to be looking for patterns
> until we have enough data to smooth this out.
> 
>

I now have results for 1M and 10M runs on the two builds (0012 and
0014), attached is a chart for relative performance plotting

  (0014 timing) / (0012 timing)

for "optimal' runs that would pick bitmapscan on their own. There's
nothing special about the config - I reduced the random_page_cost to
1.5-2.0 to reflect both machines have flash storage, etc.

Overall, the chart is pretty consistent with what I shared on Sunday.
Most of the results are fine (0014 is close to 0012 or faster), but
there's a bunch of cases that are much slower. Interestingly enough,
almost all of them are on the i5 machine, almost none of the xeon. My
guess is this is about the SSD type (SATA vs. NVMe).

Attached if table of ~50 worst regressions (by the metric above), and
it's interesting the worst regressions are with eic=0 and eic=1.

I decided to look at the first case (eic=0), and the timings are quite
stable - there are three runs for each build, with timings close to the
average (see below the table).

Attached is a script that reproduces this on both machines, but the
difference is much more significant on i5 (~5x) compared to xeon (~2x).

I haven't investigated what exactly is happening and why, hopefully the
script will allow you to reproduce this independently. I plan to take a
look, but I don't know when I'll have time for this.

FWIW if the script does not reproduce this on your machines, I might be
able to give you access to the i5 machine. Let me know.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

20 March, 21:13:05

On 3/18/24 16:19, Melanie Plageman wrote:
> On Mon, Mar 18, 2024 at 02:10:28PM +0200, Heikki Linnakangas wrote:
>> On 14/02/2024 21:42, Andres Freund wrote:
>>> On 2024-02-13 18:11:25 -0500, Melanie Plageman wrote:
>>>> patch 0004 is, I think, a bug fix. see [2].
>>>
>>> I'd not quite call it a bugfix, it's not like it leads to wrong
>>> behaviour. Seems more like an optimization. But whatever :)
>>
>> It sure looks like bug to me, albeit a very minor one. Certainly not an
>> optimization, it doesn't affect performance in any way, only what EXPLAIN
>> reports. So committed and backported that to all supported branches.
> 
> I've attached v7 rebased over this commit.
>

I've started a new set of benchmarks with v7 (on top of f69319f2f1), but
unfortunately that results in about 15% of the queries failing with:

  ERROR:  prefetch and main iterators are out of sync

Reproducing it is pretty simple (at least on my laptop). Simply apply
0001-0011, and then do this:

======================================================================
create table test_table (a bigint, b bigint, c text) with (fillfactor = 25);

insert into test_table select 10000 * random(), i, md5(random()::text)
from generate_series(1, 1000000) s(i);

create index on test_table(a);

vacuum analyze ;
checkpoint;

set work_mem = '128kB';
set effective_io_concurrency = 8;
set random_page_cost = 2;
set max_parallel_workers_per_gather = 0;

explain select * from test_table where a >= 1 and a <= 512;

explain analyze select * from test_table where a >= 1 and a <= 512;

ERROR:  prefetch and main iterators are out of sync
======================================================================

I haven't investigated this, but it seems to get broken by this patch:

  v7-0009-Make-table_scan_bitmap_next_block-async-friendly.patch

I wonder if there are some additional changes aside from the rebase.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

21 March, 17:55:14

On 3/14/24 19:16, Melanie Plageman wrote:
> ...
> 
> Attached v6 is rebased over your new commit. It also has the "fix" in
> 0010 which moves BitmapAdjustPrefetchIterator() back above
> table_scan_bitmap_next_block(). I've also updated the Streaming Read API
> commit (0013) to Thomas' v7 version from [1]. This has the update that
> we theorize should address some of the regressions in the bitmapheapscan
> streaming read user in 0014.
> 

Based on the recent discussions in this thread I've been wondering how
does the readahead setting for the device affect the behavior, so I've
modified the script to test different values for this parameter too.

Considering the bug in the v7 patch (reported yesterday elsewhere in
this thread), I had to use the v6 version for now. I don't think it
makes much difference, the important parts of the patch do not change.

The complete results are far too large to include here (multiple MBs),
so I'll only include results for a small subset of parameters (one
dataset on i5), and some scatter charts with all results to show the
overall behavior. (I only have results for 1M rows so far).

Complete results (including the raw CSV etc.) are available in a git
repo, along with jupyter notebooks that I started using for experiments
and building the other charts:

   https://github.com/tvondra/jupyterlab-projects/tree/master

If you look at the attached PDF table, the first half is for serial
execution (no parallelism), the second half is with 4 workers. And there
are 3 different readahead settings 0, 1536 and 12288 (and then different
eic values for each readahead value). The readahead values are chosen as
"disabled", 6x128kB and the default that was set by the kernel (or
wherever it comes from).

There are pretty clear patterns:

* serial runs with disabled readahead - The patch causes fairly serious
regressions (compared to master), if eic>0.

* serial runs with enabled readahead - there's still some regression for
lower matches values. Presumably, at higher values (which means larger
fraction of the table matches) the readahead kicks in, leaving the lower
values as if readahead was not enabled.

* parallel runs - The regression is much smaller, either because the
parallel workers issue requests almost as if there was readahead, or
maybe it implicitly disrupts the readahead. Not sure.

The other datasets are quite similar, feel free to check the git repo
for complete results.

One possible caveat is that maybe this affects only cases that would not
actually use bitmap scans? But if you check the attached scatter charts
(PNG), that only show results for cases where the planner would actually
pick bitmap scans on it's own, there are plenty such cases.

For the 0012 patch (chart on left), there's almost no such problem - the
results are very close to master. Similarly, there are regressions even
on the chart with readahead, but it's far less frequent/significant.

The question is whether readahead=0 is even worth worrying about? If
disabling readahead causes serious regressions even on master (clearly
visible in the PDF table), would anyone actually run with it disabled?

But I'm not sure that argument is very sound. Surely there are cases
where readahead may not detect a pattern, or where it's not supported
for some arbitrary reason (e.g. I didn't have much luck with this on
ZFS, perhaps other filesystems have similar limitations). But also what
about direct I/O? Surely that won't have readahead by kernel, right?

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

23 March, 03:22:11

On Tue, Mar 19, 2024 at 02:33:35PM +0200, Heikki Linnakangas wrote:
> On 18/03/2024 17:19, Melanie Plageman wrote:
> > I've attached v7 rebased over this commit.
> 
> If we delayed table_beginscan_bm() call further, after starting the TBM
> iterator, we could skip it altogether when the iterator is empty.
> 
> That's a further improvement, doesn't need to be part of this patch set.
> Just caught my eye while reading this.

Hmm. You mean like until after the first call to tbm_[shared]_iterate()?
AFAICT, tbm_begin_iterate() doesn't tell us anything about whether or
not the iterator is "empty". Do you mean cases when the bitmap has no
blocks in it? It seems like we should be able to tell that from the
TIDBitmap.

> 
> > v7-0003-Push-BitmapHeapScan-skip-fetch-optimization-into-.patch
> 
> I suggest to avoid the double negative with SO_CAN_SKIP_FETCH, and call the
> flag e.g. SO_NEED_TUPLE.

Agreed. Done in attached v8. Though I wondered if it was a bit weird
that the flag is set in the common case and not set in the uncommon
case...

> As yet another preliminary patch before the streaming read API, it would be
> nice to move the prefetching code to heapam.c too.

I've done this, but I can say it is not very pretty. see 0013. I had to
add a bunch of stuff to TableScanDescData and HeapScanDescData which are
only used for bitmapheapscans. I don't know if it makes the BHS
streaming read user patch easier to review, but I don't think what I
have in 0013 is committable to Postgres. Maybe there was another way I
could have approached it. Let me know what you think.

In addition to bloating the table descriptors, note that it was
difficult to avoid one semantic change -- with 0013, we no longer
prefetch or adjust prefetch target when emitting each empty tuple --
though I think this is could actually be desirable.

> What's the point of having separate table_scan_bitmap_next_block() and
> table_scan_bitmap_next_tuple() functions anymore? The AM owns the TBM
> iterator now. The executor node updates the lossy/exact page counts, but
> that's the only per-page thing it does now.

Oh, interesting. Good point. I've done this in 0015. If you like the way
it turned out, I can probably rebase this back into an earlier point in
the set and end up dropping some of the other incremental changes (e.g.
0008).

> >         /*
> >          * If this is the first scan of the underlying table, create the table
> >          * scan descriptor and begin the scan.
> >          */
> >         if (!scan)
> >         {
> >             uint32        extra_flags = 0;
> > 
> >             /*
> >              * We can potentially skip fetching heap pages if we do not need
> >              * any columns of the table, either for checking non-indexable
> >              * quals or for returning data.  This test is a bit simplistic, as
> >              * it checks the stronger condition that there's no qual or return
> >              * tlist at all. But in most cases it's probably not worth working
> >              * harder than that.
> >              */
> >             if (node->ss.ps.plan->qual == NIL && node->ss.ps.plan->targetlist == NIL)
> >                 extra_flags |= SO_CAN_SKIP_FETCH;
> > 
> >             scan = node->ss.ss_currentScanDesc = table_beginscan_bm(
> >                                                                     node->ss.ss_currentRelation,
> >                                                                     node->ss.ps.state->es_snapshot,
> >                                                                     0,
> >                                                                     NULL,
> >                                                                     extra_flags);
> >         }
> > 
> >         scan->tbmiterator = tbmiterator;
> >         scan->shared_tbmiterator = shared_tbmiterator;
> 
> How about passing the iterator as an argument to table_beginscan_bm()? You'd
> then need some other function to change the iterator on rescan, though. Not
> sure what exactly to do here, but feels that this part of the API is not
> fully thought-out. Needs comments at least, to explain who sets tbmiterator
> / shared_tbmiterator and when. For comparison, for a TID scan there's a
> separate scan_set_tidrange() table AM function. Maybe follow that example
> and introduce scan_set_tbm_iterator().

I've spent quite a bit of time playing around with the code trying to
make it less terrible than what I had before.

On rescan, we have to actually make the whole bitmap and iterator. And,
we don't have what we need to do that in table_rescan()/heap_rescan().
From what I can tell, scan_set_tidrange() is useful because it can be
called from both the beginscan and rescan functions without invoking it
directly from TidNext().

In our case, any wrapper function we wrote would basically just assign
the iterator to the scan in BitmapHeapNext().

I've reorganized this code structure a bit, so see if you like it more
now. I rebased the changes into some of the other patches, so you'll
just have to look at the result and see what you think.

> It's bit awkward to have separate tbmiterator and shared_tbmiterator fields.
> Could regular and shared iterators be merged, or wrapped under a common
> interface?

This is a good idea. I've done that in 0014. It made the code nicer, but
I just wonder if it will add too much overhead.

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

23 March, 03:26:07

On Fri, Mar 22, 2024 at 08:22:11PM -0400, Melanie Plageman wrote:
> On Tue, Mar 19, 2024 at 02:33:35PM +0200, Heikki Linnakangas wrote:
> > On 18/03/2024 17:19, Melanie Plageman wrote:
> > > I've attached v7 rebased over this commit.
> > 
> > If we delayed table_beginscan_bm() call further, after starting the TBM
> > iterator, we could skip it altogether when the iterator is empty.
> > 
> > That's a further improvement, doesn't need to be part of this patch set.
> > Just caught my eye while reading this.
> 
> Hmm. You mean like until after the first call to tbm_[shared]_iterate()?
> AFAICT, tbm_begin_iterate() doesn't tell us anything about whether or
> not the iterator is "empty". Do you mean cases when the bitmap has no
> blocks in it? It seems like we should be able to tell that from the
> TIDBitmap.
> 
> > 
> > > v7-0003-Push-BitmapHeapScan-skip-fetch-optimization-into-.patch
> > 
> > I suggest to avoid the double negative with SO_CAN_SKIP_FETCH, and call the
> > flag e.g. SO_NEED_TUPLE.
> 
> Agreed. Done in attached v8. Though I wondered if it was a bit weird
> that the flag is set in the common case and not set in the uncommon
> case...

v8 actually attached this time

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

24 March, 15:36:19


On 3/23/24 01:26, Melanie Plageman wrote:
> On Fri, Mar 22, 2024 at 08:22:11PM -0400, Melanie Plageman wrote:
>> On Tue, Mar 19, 2024 at 02:33:35PM +0200, Heikki Linnakangas wrote:
>>> On 18/03/2024 17:19, Melanie Plageman wrote:
>>>> I've attached v7 rebased over this commit.
>>>
>>> If we delayed table_beginscan_bm() call further, after starting the TBM
>>> iterator, we could skip it altogether when the iterator is empty.
>>>
>>> That's a further improvement, doesn't need to be part of this patch set.
>>> Just caught my eye while reading this.
>>
>> Hmm. You mean like until after the first call to tbm_[shared]_iterate()?
>> AFAICT, tbm_begin_iterate() doesn't tell us anything about whether or
>> not the iterator is "empty". Do you mean cases when the bitmap has no
>> blocks in it? It seems like we should be able to tell that from the
>> TIDBitmap.
>>
>>>
>>>> v7-0003-Push-BitmapHeapScan-skip-fetch-optimization-into-.patch
>>>
>>> I suggest to avoid the double negative with SO_CAN_SKIP_FETCH, and call the
>>> flag e.g. SO_NEED_TUPLE.
>>
>> Agreed. Done in attached v8. Though I wondered if it was a bit weird
>> that the flag is set in the common case and not set in the uncommon
>> case...
> 
> v8 actually attached this time

I tried to run the benchmarks with v8, but unfortunately it crashes for
me very quickly (I've only seen 0015 to crash, so I guess the bug is in
that patch).

The backtrace attached, this doesn't seem right:

(gdb) p hscan->rs_cindex
$1 = 543516018


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

crash.txt

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

24 March, 20:38:33

On Sun, Mar 24, 2024 at 01:36:19PM +0100, Tomas Vondra wrote:
> 
> 
> On 3/23/24 01:26, Melanie Plageman wrote:
> > On Fri, Mar 22, 2024 at 08:22:11PM -0400, Melanie Plageman wrote:
> >> On Tue, Mar 19, 2024 at 02:33:35PM +0200, Heikki Linnakangas wrote:
> >>> On 18/03/2024 17:19, Melanie Plageman wrote:
> >>>> I've attached v7 rebased over this commit.
> >>>
> >>> If we delayed table_beginscan_bm() call further, after starting the TBM
> >>> iterator, we could skip it altogether when the iterator is empty.
> >>>
> >>> That's a further improvement, doesn't need to be part of this patch set.
> >>> Just caught my eye while reading this.
> >>
> >> Hmm. You mean like until after the first call to tbm_[shared]_iterate()?
> >> AFAICT, tbm_begin_iterate() doesn't tell us anything about whether or
> >> not the iterator is "empty". Do you mean cases when the bitmap has no
> >> blocks in it? It seems like we should be able to tell that from the
> >> TIDBitmap.
> >>
> >>>
> >>>> v7-0003-Push-BitmapHeapScan-skip-fetch-optimization-into-.patch
> >>>
> >>> I suggest to avoid the double negative with SO_CAN_SKIP_FETCH, and call the
> >>> flag e.g. SO_NEED_TUPLE.
> >>
> >> Agreed. Done in attached v8. Though I wondered if it was a bit weird
> >> that the flag is set in the common case and not set in the uncommon
> >> case...
> > 
> > v8 actually attached this time
> 
> I tried to run the benchmarks with v8, but unfortunately it crashes for
> me very quickly (I've only seen 0015 to crash, so I guess the bug is in
> that patch).
> 
> The backtrace attached, this doesn't seem right:
> 
> (gdb) p hscan->rs_cindex
> $1 = 543516018

Thanks for reporting this! I hadn't seen it crash on my machine, so I
didn't realize that I was no longer initializing rs_cindex and
rs_ntuples on the first call to heapam_bitmap_next_tuple() (since
heapam_bitmap_next_block() wasn't being called first). I've done this in
attached v9.

I haven't had a chance yet to reproduce the regressions you saw in the
streaming read user patch or to look closely at the performance results.
I don't anticipate the streaming read user will have any performance
differences in this v9 from v6, since I haven't yet rebased in Thomas'
latest streaming read API changes nor addressed any other potential
regression sources.

I tried rebasing in Thomas' latest version today and something is
causing a crash that I have yet to figure out. v10 of this patchset will
have his latest version once I get that fixed. I wanted to share this
version with what I think is a bug fix for the crash you saw first.

- Melanie

On Sun, Mar 24, 2024 at 06:37:20PM -0400, Melanie Plageman wrote:
> On Sun, Mar 24, 2024 at 5:59 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
> >
> > BTW when you say "up to 'Make table_scan_bitmap_next_block() async
> > friendly'" do you mean including that patch, or that this is the first
> > patch that is not one of the independently useful patches.
> 
> I think the code is easier to understand with "Make
> table_scan_bitmap_next_block() async friendly". Prior to that commit,
> table_scan_bitmap_next_block() could return false even when the bitmap
> has more blocks and expects the caller to handle this and invoke it
> again. I think that interface is very confusing. The downside of the
> code in that state is that the code for prefetching is still in the
> BitmapHeapNext() code and the code for getting the current block is in
> the heap AM-specific code. I took a stab at fixing this in v9's 0013,
> but the outcome wasn't very attractive.
> 
> What I will do tomorrow is reorder and group the commits such that all
> of the commits that are useful independent of streaming read are first
> (I think 0014 and 0015 are independently valuable but they are on top
> of some things that are only useful to streaming read because they are
> more recently requested changes). I think I can actually do a bit of
> simplification in terms of how many commits there are and what is in
> each. Just to be clear, v9 is still reviewable. I am just going to go
> back and change what is included in each commit.

So, attached v10 does not include the new version of streaming read API.
I focused instead on the refactoring patches commit regrouping I
mentioned here.

I realized "Remove table_scan_bitmap_next_block()" can't easily be moved
down below "Push BitmapHeapScan prefetch code into heapam.c" because we
have to do BitmapAdjustPrefetchTarget() and
BitmapAdjustPrefetchIterator() on either side of getting the next block
(via table_scan_bitmap_next_block()).

"Push BitmapHeapScan prefetch code into heapam.c" isn't very nice
because it adds a lot of bitmapheapscan specific members to
TableScanDescData and HeapScanDescData. I thought about wrapping all of
those members in some kind of BitmapHeapScanTableState struct -- but I
don't like that because the members are spread out across
HeapScanDescData and TableScanDescData so not all of them would go in
BitmapHeapScanTableState. I could move the ones I put in
HeapScanDescData back into TableScanDescData and then wrap that in a
BitmapHeapScanTableState. I haven't done that in this version.

I did manage to move "Unify parallel and serial BitmapHeapScan iterator
interfaces" down below the line of patches which are only useful if
the streaming read user also goes in.

In attached v10, all patches up to and including "Unify parallel and
serial BitmapHeapScan iterator interfaces" (0010) are proposed for
master with or without the streaming read API.

0010 does add additional indirection and thus pointer dereferencing for
accessing the iterators, which doesn't feel good. But, it does simplify
the code.

Perhaps it is worth renaming the existing TableScanDescData->rs_parallel
(a ParallelTableScanDescData) to something like rs_seq_parallel. It is
only for sequential scans and scans of tables when building indexes but
the comments say it is for parallel scans in general. There is a similar
member in HeapScanDescData called rs_parallelworkerdata.

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

27 March, 22:37:50

On Mon, Mar 25, 2024 at 12:07:09PM -0400, Melanie Plageman wrote:
> On Sun, Mar 24, 2024 at 06:37:20PM -0400, Melanie Plageman wrote:
> > On Sun, Mar 24, 2024 at 5:59 PM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> > >
> > > BTW when you say "up to 'Make table_scan_bitmap_next_block() async
> > > friendly'" do you mean including that patch, or that this is the first
> > > patch that is not one of the independently useful patches.
> > 
> > I think the code is easier to understand with "Make
> > table_scan_bitmap_next_block() async friendly". Prior to that commit,
> > table_scan_bitmap_next_block() could return false even when the bitmap
> > has more blocks and expects the caller to handle this and invoke it
> > again. I think that interface is very confusing. The downside of the
> > code in that state is that the code for prefetching is still in the
> > BitmapHeapNext() code and the code for getting the current block is in
> > the heap AM-specific code. I took a stab at fixing this in v9's 0013,
> > but the outcome wasn't very attractive.
> > 
> > What I will do tomorrow is reorder and group the commits such that all
> > of the commits that are useful independent of streaming read are first
> > (I think 0014 and 0015 are independently valuable but they are on top
> > of some things that are only useful to streaming read because they are
> > more recently requested changes). I think I can actually do a bit of
> > simplification in terms of how many commits there are and what is in
> > each. Just to be clear, v9 is still reviewable. I am just going to go
> > back and change what is included in each commit.
> 
> So, attached v10 does not include the new version of streaming read API.
> I focused instead on the refactoring patches commit regrouping I
> mentioned here.

Attached v11 has the updated Read Stream API Thomas sent this morning
[1]. No other changes.

- Melanie

[1] https://www.postgresql.org/message-id/CA%2BhUKGJTwrS7F%3DuJPx3SeigMiQiW%2BLJaOkjGyZdCntwyMR%3DuAw%40mail.gmail.com


On 3/27/24 20:37, Melanie Plageman wrote:
> On Mon, Mar 25, 2024 at 12:07:09PM -0400, Melanie Plageman wrote:
>> On Sun, Mar 24, 2024 at 06:37:20PM -0400, Melanie Plageman wrote:
>>> On Sun, Mar 24, 2024 at 5:59 PM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>
>>>> BTW when you say "up to 'Make table_scan_bitmap_next_block() async
>>>> friendly'" do you mean including that patch, or that this is the first
>>>> patch that is not one of the independently useful patches.
>>>
>>> I think the code is easier to understand with "Make
>>> table_scan_bitmap_next_block() async friendly". Prior to that commit,
>>> table_scan_bitmap_next_block() could return false even when the bitmap
>>> has more blocks and expects the caller to handle this and invoke it
>>> again. I think that interface is very confusing. The downside of the
>>> code in that state is that the code for prefetching is still in the
>>> BitmapHeapNext() code and the code for getting the current block is in
>>> the heap AM-specific code. I took a stab at fixing this in v9's 0013,
>>> but the outcome wasn't very attractive.
>>>
>>> What I will do tomorrow is reorder and group the commits such that all
>>> of the commits that are useful independent of streaming read are first
>>> (I think 0014 and 0015 are independently valuable but they are on top
>>> of some things that are only useful to streaming read because they are
>>> more recently requested changes). I think I can actually do a bit of
>>> simplification in terms of how many commits there are and what is in
>>> each. Just to be clear, v9 is still reviewable. I am just going to go
>>> back and change what is included in each commit.
>>
>> So, attached v10 does not include the new version of streaming read API.
>> I focused instead on the refactoring patches commit regrouping I
>> mentioned here.
> 
> Attached v11 has the updated Read Stream API Thomas sent this morning
> [1]. No other changes.
> 

I think there's some sort of bug, triggering this assert in heapam

  Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);

I haven't looked for the root cause, and it's not exactly deterministic,
but try this:

  create table t (a int, b text);

  insert into t select 10000 * random(), md5(i::text)
    from generate_series(1,10000000) s(i);^C

  create index on t (a);

  explain analyze select * from t where a = 200;
  explain analyze select * from t where a < 200;

and then vary the condition a bit (different values, inequalities,
etc.). For me it hits the assert in a couple tries.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

bt.txt

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Thomas Munro

Date:

29 March, 04:12:48

On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
> I think there's some sort of bug, triggering this assert in heapam
>
>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);

Thanks for the repro.  I can't seem to reproduce it (still trying) but
I assume this is with Melanie's v11 patch set which had
v11-0016-v10-Read-Stream-API.patch.

Would you mind removing that commit and instead applying the v13
stream_read.c patches[1]?  v10 stream_read.c was a little confused
about random I/O combining, which I fixed with a small adjustment to
the conditions for the "if" statement right at the end of
read_stream_look_ahead().  Sorry about that.  The fixed version, with
eic=4, with your test query using WHERE a < a, ends its scan with:

...
posix_fadvise(32,0x28aee000,0x4000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0@4\M-5:\0\0\^D\0\M-x\^A"...,40960,0x28acc000) = 40960 (0xa000)
posix_fadvise(32,0x28af4000,0x4000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0\^XC\M-6:\0\0\^D\0\M-x"...,32768,0x28ad8000) = 32768 (0x8000)
posix_fadvise(32,0x28afc000,0x4000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0\M-XQ\M-7:\0\0\^D\0\M-x"...,24576,0x28ae4000) = 24576 (0x6000)
posix_fadvise(32,0x28b02000,0x8000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0\M^@3\M-8:\0\0\^D\0\M-x"...,16384,0x28aee000) = 16384 (0x4000)
pread(32,"\0\0\0\0\M-`\M-:\M-8:\0\0\^D\0"...,16384,0x28af4000) = 16384 (0x4000)
pread(32,"\0\0\0\0po\M-9:\0\0\^D\0\M-x\^A"...,16384,0x28afc000) = 16384 (0x4000)
pread(32,"\0\0\0\0\M-P\M-v\M-9:\0\0\^D\0"...,32768,0x28b02000) = 32768 (0x8000)

In other words it's able to coalesce, but v10 was a bit b0rked in that
respect and wouldn't do as well at that.  Then if you set
io_combine_limit = 1, it looks more like master, eg lots of little
reads, but not as many fadvises as master because of sequential
access:

...
posix_fadvise(32,0x28af4000,0x2000,POSIX_FADV_WILLNEED) = 0 (0x0) -+
pread(32,...,8192,0x28ae8000) = 8192 (0x2000)                      |
pread(32,...,8192,0x28aee000) = 8192 (0x2000)                      |
posix_fadvise(32,0x28afc000,0x2000,POSIX_FADV_WILLNEED) = 0 (0x0) ---+
pread(32,...,8192,0x28af0000) = 8192 (0x2000)                      | |
pread(32,...,8192,0x28af4000) = 8192 (0x2000) <--------------------+ |
posix_fadvise(32,0x28b02000,0x2000,POSIX_FADV_WILLNEED) = 0 (0x0) -----+
pread(32,...,8192,0x28af6000) = 8192 (0x2000)                        | |
pread(32,...,8192,0x28afc000) = 8192 (0x2000) <----------------------+ |
pread(32,...,8192,0x28afe000) = 8192 (0x2000)  }-- no advice           |
pread(32,...,8192,0x28b02000) = 8192 (0x2000) <------------------------+
pread(32,...,8192,0x28b04000) = 8192 (0x2000)  }
pread(32,...,8192,0x28b06000) = 8192 (0x2000)  }-- no advice
pread(32,...,8192,0x28b08000) = 8192 (0x2000)  }

It becomes slightly less eager to start I/Os as soon as
io_combine_limit > 1, because when it has hit max_ios, if ... <thinks>
yeah if the average block that it can combine is bigger than 4, an
arbitrary number from:

    max_pinned_buffers = Max(max_ios * 4, io_combine_limit);

.... then it can run out of look ahead window before it can reach
max_ios (aka eic), so that's a kind of arbitrary/bogus I/O depth
constraint, which is another way of saying what I was saying earlier:
maybe it just needs more distance.  So let's see the average combined
I/O length in your test query... for me it works out to 27,169 bytes.
But I think there must be times when it runs out of window due to
clustering.  So you could also try increasing that 4->8 to see what
happens to performance.

[1] https://www.postgresql.org/message-id/CA%2BhUKG%2B5UofvseJWv6YqKmuc_%3Drguc7VqKcNEG1eawKh3MzHXQ%40mail.gmail.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

29 March, 14:05:15


On 3/29/24 02:12, Thomas Munro wrote:
> On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>> I think there's some sort of bug, triggering this assert in heapam
>>
>>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
> 
> Thanks for the repro.  I can't seem to reproduce it (still trying) but
> I assume this is with Melanie's v11 patch set which had
> v11-0016-v10-Read-Stream-API.patch.
> 
> Would you mind removing that commit and instead applying the v13
> stream_read.c patches[1]?  v10 stream_read.c was a little confused
> about random I/O combining, which I fixed with a small adjustment to
> the conditions for the "if" statement right at the end of
> read_stream_look_ahead().  Sorry about that.  The fixed version, with
> eic=4, with your test query using WHERE a < a, ends its scan with:
> 

I'll give that a try. Unfortunately unfortunately the v11 still has the
problem I reported about a week ago:

  ERROR:  prefetch and main iterators are out of sync

So I can't run the full benchmarks :-( but master vs. streaming read API
should work, I think.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Thomas Munro

Date:

29 March, 16:36:08

On Sat, Mar 30, 2024 at 12:17 AM Thomas Munro <thomas.munro@gmail.com> wrote:
> eic unpatched patched
> 0        4172    9572
> 1       30846   10376
> 2       18435    5562
> 4       18980    3503
> 8       18980    2680
> 16      18976    3233

... but the patched version gets down to a low number for eic=0 too if
you turn up the blockdev --setra so that it also gets Linux RA
treatment, making it the clear winner on all eic settings.  Patched
doesn't improve.  So, for low IOPS storage at least, when you're on
the borderline between random and sequential, ie bitmap with a lot of
1s in it, it seems there are cases where patched doesn't trigger Linux
RA but unpatched does, and you can tune your way out of that, and then
there are cases where the IOPS limit is reached due to small reads,
but patched does better because of larger I/Os that are likely under
the same circumstances.  Does that make sense?

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

29 March, 18:52:58

On 3/29/24 14:36, Thomas Munro wrote:
> On Sat, Mar 30, 2024 at 12:17 AM Thomas Munro <thomas.munro@gmail.com> wrote:
>> eic unpatched patched
>> 0        4172    9572
>> 1       30846   10376
>> 2       18435    5562
>> 4       18980    3503
>> 8       18980    2680
>> 16      18976    3233
> 
> ... but the patched version gets down to a low number for eic=0 too if
> you turn up the blockdev --setra so that it also gets Linux RA
> treatment, making it the clear winner on all eic settings.  Patched
> doesn't improve.  So, for low IOPS storage at least, when you're on
> the borderline between random and sequential, ie bitmap with a lot of
> 1s in it, it seems there are cases where patched doesn't trigger Linux
> RA but unpatched does, and you can tune your way out of that, and then
> there are cases where the IOPS limit is reached due to small reads,
> but patched does better because of larger I/Os that are likely under
> the same circumstances.  Does that make sense?

I think you meant "unpatched version gets down" in the first sentence,
right? Still, it seems clear this changes the interaction with readahead
done by the kernel.

However, you seem to focus only on eic=0/eic=1 cases, but IIRC that was
just an example. There are regression with higher eic values too.

I do have some early results from the benchmarks - it's just from the
NVMe machine, with 1M tables (~300MB), and it's just one incomplete run
(so there might be some noise etc.).

Attached is a PDF with charts for different subsets of the runs:

- optimal (would optimizer pick bitmapscan or not)
- readahead (yes/no)
- workers (serial vs. 4 workers)
- combine limit (8kB / 128kB)

The most interesting cases are first two rows, i.e. optimal plans.
Either with readahead enabled (first row) or disabled (second row).

Two observations:

* The combine limit seems to have negligible impact. There's no visible
difference between combine_limit=8kB and 128kB.

* Parallel queries seem to work about the same as master (especially for
optimal cases, but even for not optimal ones).

The optimal plans with kernel readahead (two charts in the first row)
look fairly good. There are a couple regressed cases, but a bunch of
faster ones too.

The optimal plans without kernel read ahead (two charts in the second
row) perform pretty poorly - there are massive regressions. But I think
the obvious reason is that the streaming read API skips prefetches for
sequential access patterns, relying on kernel to do the readahead. But
if the kernel readahead is disabled for the device, that obviously can't
happen ...

I think the question is how much we can (want to) rely on the readahead
to be done by the kernel. Maybe there should be some flag to force
issuing fadvise even for sequential patterns, perhaps at the tablespace
level? I don't recall seeing a system with disabled readahead, but I'm
sure there are cases where it may not really work - it clearly can't
work with direct I/O, but I've also not been very successful with
prefetching on ZFS.

The non-optimal plans (second half of the charts) shows about the same
behavior, but the regressions are more frequent / significant.

I'm also attaching results for the 5k "optimal" runs, showing the timing
for master and patched build, sorted by (patched/master). The most
significant regressions are with readahead=0, but if you filter that out
you'll see the regressions affect a mix of data sets, not just the
uniformly random data used as example before.

On 3/29/24 12:17, Thomas Munro wrote:
> ...
> On the other hand this might be a pretty unusual data distribution.
> People who store random numbers or hashes or whatever probably don't
> really search for ranges of them (unless they're trying to mine
> bitcoins in SQL).  I dunno.  Maybe we need more realistic tests, or
> maybe we're just discovering all the things that are bad about the
> pre-existing code.

I certainly admit the data sets are synthetic and perhaps adversarial.
My intent was to cover a wide range of data sets, to trigger even less
common cases. It's certainly up to debate how serious the regressions on
those data sets are in practice, I'm not suggesting "this strange data
set makes it slower than master, so we can't commit this".

But I'd also point that what matters is the access pattern, not the
exact query generating it. I agree people probably don't do random
numbers or hashes with range conditions, but that's irrelevant - what
it's all about the page access pattern. If you have IPv4 addresses and
query that, that's likely going to be pretty random, for example.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Fri, Mar 29, 2024 at 12:05:15PM +0100, Tomas Vondra wrote:
> 
> 
> On 3/29/24 02:12, Thomas Munro wrote:
> > On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >> I think there's some sort of bug, triggering this assert in heapam
> >>
> >>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
> > 
> > Thanks for the repro.  I can't seem to reproduce it (still trying) but
> > I assume this is with Melanie's v11 patch set which had
> > v11-0016-v10-Read-Stream-API.patch.
> > 
> > Would you mind removing that commit and instead applying the v13
> > stream_read.c patches[1]?  v10 stream_read.c was a little confused
> > about random I/O combining, which I fixed with a small adjustment to
> > the conditions for the "if" statement right at the end of
> > read_stream_look_ahead().  Sorry about that.  The fixed version, with
> > eic=4, with your test query using WHERE a < a, ends its scan with:
> > 
> 
> I'll give that a try. Unfortunately unfortunately the v11 still has the
> problem I reported about a week ago:
> 
>   ERROR:  prefetch and main iterators are out of sync
> 
> So I can't run the full benchmarks :-( but master vs. streaming read API
> should work, I think.

Odd, I didn't notice you reporting this ERROR popping up. Now that I
take a look, v11 (at least, maybe also v10) had this very sill mistake:

  if (scan->bm_parallel == NULL &&
      scan->rs_pf_bhs_iterator &&
      hscan->pfblockno > hscan->rs_base.blockno)
       elog(ERROR, "prefetch and main iterators are out of sync");

It errors out if the prefetch block is ahead of the current block --
which is the opposite of what we want. I've fixed this in attached v12.

This version also has v13 of the streaming read API. I noticed one
mistake in my bitmapheap scan streaming read user -- it freed the
streaming read object at the wrong time. I don't know if this was
causing any other issues, but it at least is fixed in this version.

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

04 April, 01:57:59

On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
> On Fri, Mar 29, 2024 at 12:05:15PM +0100, Tomas Vondra wrote:
> > 
> > 
> > On 3/29/24 02:12, Thomas Munro wrote:
> > > On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
> > > <tomas.vondra@enterprisedb.com> wrote:
> > >> I think there's some sort of bug, triggering this assert in heapam
> > >>
> > >>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
> > > 
> > > Thanks for the repro.  I can't seem to reproduce it (still trying) but
> > > I assume this is with Melanie's v11 patch set which had
> > > v11-0016-v10-Read-Stream-API.patch.
> > > 
> > > Would you mind removing that commit and instead applying the v13
> > > stream_read.c patches[1]?  v10 stream_read.c was a little confused
> > > about random I/O combining, which I fixed with a small adjustment to
> > > the conditions for the "if" statement right at the end of
> > > read_stream_look_ahead().  Sorry about that.  The fixed version, with
> > > eic=4, with your test query using WHERE a < a, ends its scan with:
> > > 
> > 
> > I'll give that a try. Unfortunately unfortunately the v11 still has the
> > problem I reported about a week ago:
> > 
> >   ERROR:  prefetch and main iterators are out of sync
> > 
> > So I can't run the full benchmarks :-( but master vs. streaming read API
> > should work, I think.
> 
> Odd, I didn't notice you reporting this ERROR popping up. Now that I
> take a look, v11 (at least, maybe also v10) had this very sill mistake:
> 
>   if (scan->bm_parallel == NULL &&
>       scan->rs_pf_bhs_iterator &&
>       hscan->pfblockno > hscan->rs_base.blockno)
>        elog(ERROR, "prefetch and main iterators are out of sync");
> 
> It errors out if the prefetch block is ahead of the current block --
> which is the opposite of what we want. I've fixed this in attached v12.
> 
> This version also has v13 of the streaming read API. I noticed one
> mistake in my bitmapheap scan streaming read user -- it freed the
> streaming read object at the wrong time. I don't know if this was
> causing any other issues, but it at least is fixed in this version.

Attached v13 is rebased over master (which includes the streaming read
API now). I also reset the streaming read object on rescan instead of
creating a new one each time.

I don't know how much chance any of this has of going in to 17 now, but
I thought I would start looking into the regression repro Tomas provided
in [1].

I'm also not sure if I should try and group the commits into fewer
commits now or wait until I have some idea of whether or not the
approach in 0013 and 0014 is worth pursuing.

- Melanie

[1] https://www.postgresql.org/message-id/5d5954ed-6f43-4f1a-8e19-ece75b2b7362%40enterprisedb.com

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

04 April, 17:35:45

On 4/4/24 00:57, Melanie Plageman wrote:
> On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
>> On Fri, Mar 29, 2024 at 12:05:15PM +0100, Tomas Vondra wrote:
>>>
>>>
>>> On 3/29/24 02:12, Thomas Munro wrote:
>>>> On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
>>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>> I think there's some sort of bug, triggering this assert in heapam
>>>>>
>>>>>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
>>>>
>>>> Thanks for the repro.  I can't seem to reproduce it (still trying) but
>>>> I assume this is with Melanie's v11 patch set which had
>>>> v11-0016-v10-Read-Stream-API.patch.
>>>>
>>>> Would you mind removing that commit and instead applying the v13
>>>> stream_read.c patches[1]?  v10 stream_read.c was a little confused
>>>> about random I/O combining, which I fixed with a small adjustment to
>>>> the conditions for the "if" statement right at the end of
>>>> read_stream_look_ahead().  Sorry about that.  The fixed version, with
>>>> eic=4, with your test query using WHERE a < a, ends its scan with:
>>>>
>>>
>>> I'll give that a try. Unfortunately unfortunately the v11 still has the
>>> problem I reported about a week ago:
>>>
>>>   ERROR:  prefetch and main iterators are out of sync
>>>
>>> So I can't run the full benchmarks :-( but master vs. streaming read API
>>> should work, I think.
>>
>> Odd, I didn't notice you reporting this ERROR popping up. Now that I
>> take a look, v11 (at least, maybe also v10) had this very sill mistake:
>>
>>   if (scan->bm_parallel == NULL &&
>>       scan->rs_pf_bhs_iterator &&
>>       hscan->pfblockno > hscan->rs_base.blockno)
>>        elog(ERROR, "prefetch and main iterators are out of sync");
>>
>> It errors out if the prefetch block is ahead of the current block --
>> which is the opposite of what we want. I've fixed this in attached v12.
>>
>> This version also has v13 of the streaming read API. I noticed one
>> mistake in my bitmapheap scan streaming read user -- it freed the
>> streaming read object at the wrong time. I don't know if this was
>> causing any other issues, but it at least is fixed in this version.
> 
> Attached v13 is rebased over master (which includes the streaming read
> API now). I also reset the streaming read object on rescan instead of
> creating a new one each time.
> 
> I don't know how much chance any of this has of going in to 17 now, but
> I thought I would start looking into the regression repro Tomas provided
> in [1].
> 

My personal opinion is that we should try to get in as many of the the
refactoring patches as possible, but I think it's probably too late for
the actual switch to the streaming API.

If someone else feels like committing that part, I won't stand in the
way, but I'm not quite convinced it won't cause regressions. Maybe it's
OK but I'd need more time to do more tests, collect data, and so on. And
I don't think we have that, especially considering we'd still need to
commit the other parts first.

> I'm also not sure if I should try and group the commits into fewer
> commits now or wait until I have some idea of whether or not the
> approach in 0013 and 0014 is worth pursuing.
> 

You mean whether to pursue the approach in general, or for v17? I think
it looks like the right approach, but for v17 see above :-(

As for merging, I wouldn't do that. I looked at the commits and while
some of them seem somewhat "trivial", I really like how you organized
the commits, and kept those that just "move" code around, and those that
actually change stuff. It's much easier to understand, IMO.

I went through the first ~10 commits, and added some review - either as
a separate commit, when possible, in the code as XXX comment, and also
in the commit message. The code tweaks are utterly trivial (whitespace
or indentation to make the line shorter). It shouldn't take much time to
deal with those, I think.

I think the main focus should be updating the commit messages. If it was
only a single patch, I'd probably try to write the messages myself, but
with this many patches it'd be great if you could update those and I'll
review that before commit.

I always struggle with writing commit messages myself, and it takes me
ages to write a good one (well, I think the message is good, but who
knows ...). But I think a good message should be concise enough to
explain what and why it's done. It may reference a thread for all the
gory details, but the basic reasoning should be in the commit message.
For example the message for "BitmapPrefetch use prefetch block recheck
for skip fetch" now says that it "makes more sense to do X" but does not
really say why that's the case. The linked message does, but it'd be
good to have that in the message (because how would I know how much of
the thread to read?).

Also, it'd be very helpful if you could update the author & reviewed-by
fields. I'll review those before commit, ofc, but I admit I lost track
of who reviewed which part.

I'd focus on the first ~8-9 commits or so for now, we can commit more if
things go reasonably well.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

05 April, 11:06:34

On Thu, Apr 04, 2024 at 04:35:45PM +0200, Tomas Vondra wrote:
> 
> 
> On 4/4/24 00:57, Melanie Plageman wrote:
> > On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
> >> On Fri, Mar 29, 2024 at 12:05:15PM +0100, Tomas Vondra wrote:
> >>>
> >>>
> >>> On 3/29/24 02:12, Thomas Munro wrote:
> >>>> On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
> >>>> <tomas.vondra@enterprisedb.com> wrote:
> >>>>> I think there's some sort of bug, triggering this assert in heapam
> >>>>>
> >>>>>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
> >>>>
> >>>> Thanks for the repro.  I can't seem to reproduce it (still trying) but
> >>>> I assume this is with Melanie's v11 patch set which had
> >>>> v11-0016-v10-Read-Stream-API.patch.
> >>>>
> >>>> Would you mind removing that commit and instead applying the v13
> >>>> stream_read.c patches[1]?  v10 stream_read.c was a little confused
> >>>> about random I/O combining, which I fixed with a small adjustment to
> >>>> the conditions for the "if" statement right at the end of
> >>>> read_stream_look_ahead().  Sorry about that.  The fixed version, with
> >>>> eic=4, with your test query using WHERE a < a, ends its scan with:
> >>>>
> >>>
> >>> I'll give that a try. Unfortunately unfortunately the v11 still has the
> >>> problem I reported about a week ago:
> >>>
> >>>   ERROR:  prefetch and main iterators are out of sync
> >>>
> >>> So I can't run the full benchmarks :-( but master vs. streaming read API
> >>> should work, I think.
> >>
> >> Odd, I didn't notice you reporting this ERROR popping up. Now that I
> >> take a look, v11 (at least, maybe also v10) had this very sill mistake:
> >>
> >>   if (scan->bm_parallel == NULL &&
> >>       scan->rs_pf_bhs_iterator &&
> >>       hscan->pfblockno > hscan->rs_base.blockno)
> >>        elog(ERROR, "prefetch and main iterators are out of sync");
> >>
> >> It errors out if the prefetch block is ahead of the current block --
> >> which is the opposite of what we want. I've fixed this in attached v12.
> >>
> >> This version also has v13 of the streaming read API. I noticed one
> >> mistake in my bitmapheap scan streaming read user -- it freed the
> >> streaming read object at the wrong time. I don't know if this was
> >> causing any other issues, but it at least is fixed in this version.
> > 
> > Attached v13 is rebased over master (which includes the streaming read
> > API now). I also reset the streaming read object on rescan instead of
> > creating a new one each time.
> > 
> > I don't know how much chance any of this has of going in to 17 now, but
> > I thought I would start looking into the regression repro Tomas provided
> > in [1].
> > 
> 
> My personal opinion is that we should try to get in as many of the the
> refactoring patches as possible, but I think it's probably too late for
> the actual switch to the streaming API.

Cool. In the attached v15, I have dropped all commits that are related
to the streaming read API and included *only* commits that are
beneficial to master. A few of the commits are merged or reordered as
well.

While going through the commits with this new goal in mind (forget about
the streaming read API for now), I realized that it doesn't make much
sense to just eliminate the layering violation for the current block and
leave it there for the prefetch block. I had de-prioritized solving this
when I thought we would just delete the prefetch code and replace it
with the streaming read.

Now that we aren't doing that, I've spent the day trying to resolve the
issues with pushing the prefetch code into heapam.c that I cited in [1].
0010 - 0013 are the result of this. They are not very polished yet and
need more cleanup and review (especially 0011, which is probably too
large), but I am happy with the solution I came up with.

Basically, there are too many members needed for bitmap heap scan to put
them all in the HeapScanDescData (don't want to bloat it). So, I've made
a new BitmapHeapScanDescData and associated begin/rescan/end() functions

In the end, with all patches applied, BitmapHeapNext() loops invoking
table_scan_bitmap_next_tuple() and table AMs can implement that however
they choose.

> > I'm also not sure if I should try and group the commits into fewer
> > commits now or wait until I have some idea of whether or not the
> > approach in 0013 and 0014 is worth pursuing.
> > 
> 
> You mean whether to pursue the approach in general, or for v17? I think
> it looks like the right approach, but for v17 see above :-(
> 
> As for merging, I wouldn't do that. I looked at the commits and while
> some of them seem somewhat "trivial", I really like how you organized
> the commits, and kept those that just "move" code around, and those that
> actually change stuff. It's much easier to understand, IMO.
> 
> I went through the first ~10 commits, and added some review - either as
> a separate commit, when possible, in the code as XXX comment, and also
> in the commit message. The code tweaks are utterly trivial (whitespace
> or indentation to make the line shorter). It shouldn't take much time to
> deal with those, I think.

Attached v15 incorporates your v14-0002-review.

For your v14-0008-review, I actually ended up removing that commit
because once I removed everything that was for streaming read API, it
became redundant with another commit.

For your v14-0010-review, we actually can't easily get rid of those
local variables because we make the iterators before we make the scan
descriptors and the commit following that commit moves the iterators
from the BitmapHeapScanState to the scan descriptor.

> I think the main focus should be updating the commit messages. If it was
> only a single patch, I'd probably try to write the messages myself, but
> with this many patches it'd be great if you could update those and I'll
> review that before commit.

I did my best to update the commit messages to be less specific and more
focused on "why should I care". I found myself wanting to explain why I
implemented something the way I did and then getting back into the
implementation details again. I'm not sure if I suceeded in having less
details and more substance.

> I always struggle with writing commit messages myself, and it takes me
> ages to write a good one (well, I think the message is good, but who
> knows ...). But I think a good message should be concise enough to
> explain what and why it's done. It may reference a thread for all the
> gory details, but the basic reasoning should be in the commit message.
> For example the message for "BitmapPrefetch use prefetch block recheck
> for skip fetch" now says that it "makes more sense to do X" but does not
> really say why that's the case. The linked message does, but it'd be
> good to have that in the message (because how would I know how much of
> the thread to read?).

I fixed that particular one. I tried to take that feedback and apply it
to other commit messages. I don't know how successful I was...

> Also, it'd be very helpful if you could update the author & reviewed-by
> fields. I'll review those before commit, ofc, but I admit I lost track
> of who reviewed which part.

I have updated reviewers. I didn't add reviewers on the ones that
haven't really been reviewed yet.

> I'd focus on the first ~8-9 commits or so for now, we can commit more if
> things go reasonably well.

Sounds good. I will spend cleanup time on 0010-0013 tomorrow but would
love to know if you agree with the direction before I spend more time.

- Melanie

[1] https://www.postgresql.org/message-id/20240323002211.on5vb5ulk6lsdb2u%40liskov

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

06 April, 02:53:30

On Fri, Apr 05, 2024 at 04:06:34AM -0400, Melanie Plageman wrote:
> On Thu, Apr 04, 2024 at 04:35:45PM +0200, Tomas Vondra wrote:
> > 
> > 
> > On 4/4/24 00:57, Melanie Plageman wrote:
> > > On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
> > >> On Fri, Mar 29, 2024 at 12:05:15PM +0100, Tomas Vondra wrote:
> > >>>
> > >>>
> > >>> On 3/29/24 02:12, Thomas Munro wrote:
> > >>>> On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
> > >>>> <tomas.vondra@enterprisedb.com> wrote:
> > >>>>> I think there's some sort of bug, triggering this assert in heapam
> > >>>>>
> > >>>>>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
> > >>>>
> > >>>> Thanks for the repro.  I can't seem to reproduce it (still trying) but
> > >>>> I assume this is with Melanie's v11 patch set which had
> > >>>> v11-0016-v10-Read-Stream-API.patch.
> > >>>>
> > >>>> Would you mind removing that commit and instead applying the v13
> > >>>> stream_read.c patches[1]?  v10 stream_read.c was a little confused
> > >>>> about random I/O combining, which I fixed with a small adjustment to
> > >>>> the conditions for the "if" statement right at the end of
> > >>>> read_stream_look_ahead().  Sorry about that.  The fixed version, with
> > >>>> eic=4, with your test query using WHERE a < a, ends its scan with:
> > >>>>
> > >>>
> > >>> I'll give that a try. Unfortunately unfortunately the v11 still has the
> > >>> problem I reported about a week ago:
> > >>>
> > >>>   ERROR:  prefetch and main iterators are out of sync
> > >>>
> > >>> So I can't run the full benchmarks :-( but master vs. streaming read API
> > >>> should work, I think.
> > >>
> > >> Odd, I didn't notice you reporting this ERROR popping up. Now that I
> > >> take a look, v11 (at least, maybe also v10) had this very sill mistake:
> > >>
> > >>   if (scan->bm_parallel == NULL &&
> > >>       scan->rs_pf_bhs_iterator &&
> > >>       hscan->pfblockno > hscan->rs_base.blockno)
> > >>        elog(ERROR, "prefetch and main iterators are out of sync");
> > >>
> > >> It errors out if the prefetch block is ahead of the current block --
> > >> which is the opposite of what we want. I've fixed this in attached v12.
> > >>
> > >> This version also has v13 of the streaming read API. I noticed one
> > >> mistake in my bitmapheap scan streaming read user -- it freed the
> > >> streaming read object at the wrong time. I don't know if this was
> > >> causing any other issues, but it at least is fixed in this version.
> > > 
> > > Attached v13 is rebased over master (which includes the streaming read
> > > API now). I also reset the streaming read object on rescan instead of
> > > creating a new one each time.
> > > 
> > > I don't know how much chance any of this has of going in to 17 now, but
> > > I thought I would start looking into the regression repro Tomas provided
> > > in [1].
> > > 
> > 
> > My personal opinion is that we should try to get in as many of the the
> > refactoring patches as possible, but I think it's probably too late for
> > the actual switch to the streaming API.
> 
> Cool. In the attached v15, I have dropped all commits that are related
> to the streaming read API and included *only* commits that are
> beneficial to master. A few of the commits are merged or reordered as
> well.
> 
> While going through the commits with this new goal in mind (forget about
> the streaming read API for now), I realized that it doesn't make much
> sense to just eliminate the layering violation for the current block and
> leave it there for the prefetch block. I had de-prioritized solving this
> when I thought we would just delete the prefetch code and replace it
> with the streaming read.
> 
> Now that we aren't doing that, I've spent the day trying to resolve the
> issues with pushing the prefetch code into heapam.c that I cited in [1].
> 0010 - 0013 are the result of this. They are not very polished yet and
> need more cleanup and review (especially 0011, which is probably too
> large), but I am happy with the solution I came up with.
> 
> Basically, there are too many members needed for bitmap heap scan to put
> them all in the HeapScanDescData (don't want to bloat it). So, I've made
> a new BitmapHeapScanDescData and associated begin/rescan/end() functions
> 
> In the end, with all patches applied, BitmapHeapNext() loops invoking
> table_scan_bitmap_next_tuple() and table AMs can implement that however
> they choose.
> 
> > > I'm also not sure if I should try and group the commits into fewer
> > > commits now or wait until I have some idea of whether or not the
> > > approach in 0013 and 0014 is worth pursuing.
> > > 
> > 
> > You mean whether to pursue the approach in general, or for v17? I think
> > it looks like the right approach, but for v17 see above :-(
> > 
> > As for merging, I wouldn't do that. I looked at the commits and while
> > some of them seem somewhat "trivial", I really like how you organized
> > the commits, and kept those that just "move" code around, and those that
> > actually change stuff. It's much easier to understand, IMO.
> > 
> > I went through the first ~10 commits, and added some review - either as
> > a separate commit, when possible, in the code as XXX comment, and also
> > in the commit message. The code tweaks are utterly trivial (whitespace
> > or indentation to make the line shorter). It shouldn't take much time to
> > deal with those, I think.
> 
> Attached v15 incorporates your v14-0002-review.
> 
> For your v14-0008-review, I actually ended up removing that commit
> because once I removed everything that was for streaming read API, it
> became redundant with another commit.
> 
> For your v14-0010-review, we actually can't easily get rid of those
> local variables because we make the iterators before we make the scan
> descriptors and the commit following that commit moves the iterators
> from the BitmapHeapScanState to the scan descriptor.
> 
> > I think the main focus should be updating the commit messages. If it was
> > only a single patch, I'd probably try to write the messages myself, but
> > with this many patches it'd be great if you could update those and I'll
> > review that before commit.
> 
> I did my best to update the commit messages to be less specific and more
> focused on "why should I care". I found myself wanting to explain why I
> implemented something the way I did and then getting back into the
> implementation details again. I'm not sure if I suceeded in having less
> details and more substance.
> 
> > I always struggle with writing commit messages myself, and it takes me
> > ages to write a good one (well, I think the message is good, but who
> > knows ...). But I think a good message should be concise enough to
> > explain what and why it's done. It may reference a thread for all the
> > gory details, but the basic reasoning should be in the commit message.
> > For example the message for "BitmapPrefetch use prefetch block recheck
> > for skip fetch" now says that it "makes more sense to do X" but does not
> > really say why that's the case. The linked message does, but it'd be
> > good to have that in the message (because how would I know how much of
> > the thread to read?).
> 
> I fixed that particular one. I tried to take that feedback and apply it
> to other commit messages. I don't know how successful I was...
> 
> > Also, it'd be very helpful if you could update the author & reviewed-by
> > fields. I'll review those before commit, ofc, but I admit I lost track
> > of who reviewed which part.
> 
> I have updated reviewers. I didn't add reviewers on the ones that
> haven't really been reviewed yet.
> 
> > I'd focus on the first ~8-9 commits or so for now, we can commit more if
> > things go reasonably well.
> 
> Sounds good. I will spend cleanup time on 0010-0013 tomorrow but would
> love to know if you agree with the direction before I spend more time.

In attached v16, I've split out 0010-0013 into 0011-0017. I think it is
much easier to understand.

While I was doing that, I realized that I should remove the call to
table_rescan() from ExecReScanBitmapHeapScan() and just rely on the new
table_rescan_bm() invoked from BitmapHeapNext(). That is done in the
attached.

0010-0018 still need comments updated but I focused on getting the split
out, reviewable version of them ready. I'll add comments (especially to
0011 table AM functions) tomorrow. I also have to double-check if I
should add any asserts for table AMs about having implemented all of the
new begin/re/endscan() functions.

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

06 April, 03:51:45


On 4/6/24 01:53, Melanie Plageman wrote:
> On Fri, Apr 05, 2024 at 04:06:34AM -0400, Melanie Plageman wrote:
>> On Thu, Apr 04, 2024 at 04:35:45PM +0200, Tomas Vondra wrote:
>>>
>>>
>>> On 4/4/24 00:57, Melanie Plageman wrote:
>>>> On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
>>>>> On Fri, Mar 29, 2024 at 12:05:15PM +0100, Tomas Vondra wrote:
>>>>>>
>>>>>>
>>>>>> On 3/29/24 02:12, Thomas Munro wrote:
>>>>>>> On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
>>>>>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>>>>> I think there's some sort of bug, triggering this assert in heapam
>>>>>>>>
>>>>>>>>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);
>>>>>>>
>>>>>>> Thanks for the repro.  I can't seem to reproduce it (still trying) but
>>>>>>> I assume this is with Melanie's v11 patch set which had
>>>>>>> v11-0016-v10-Read-Stream-API.patch.
>>>>>>>
>>>>>>> Would you mind removing that commit and instead applying the v13
>>>>>>> stream_read.c patches[1]?  v10 stream_read.c was a little confused
>>>>>>> about random I/O combining, which I fixed with a small adjustment to
>>>>>>> the conditions for the "if" statement right at the end of
>>>>>>> read_stream_look_ahead().  Sorry about that.  The fixed version, with
>>>>>>> eic=4, with your test query using WHERE a < a, ends its scan with:
>>>>>>>
>>>>>>
>>>>>> I'll give that a try. Unfortunately unfortunately the v11 still has the
>>>>>> problem I reported about a week ago:
>>>>>>
>>>>>>   ERROR:  prefetch and main iterators are out of sync
>>>>>>
>>>>>> So I can't run the full benchmarks :-( but master vs. streaming read API
>>>>>> should work, I think.
>>>>>
>>>>> Odd, I didn't notice you reporting this ERROR popping up. Now that I
>>>>> take a look, v11 (at least, maybe also v10) had this very sill mistake:
>>>>>
>>>>>   if (scan->bm_parallel == NULL &&
>>>>>       scan->rs_pf_bhs_iterator &&
>>>>>       hscan->pfblockno > hscan->rs_base.blockno)
>>>>>        elog(ERROR, "prefetch and main iterators are out of sync");
>>>>>
>>>>> It errors out if the prefetch block is ahead of the current block --
>>>>> which is the opposite of what we want. I've fixed this in attached v12.
>>>>>
>>>>> This version also has v13 of the streaming read API. I noticed one
>>>>> mistake in my bitmapheap scan streaming read user -- it freed the
>>>>> streaming read object at the wrong time. I don't know if this was
>>>>> causing any other issues, but it at least is fixed in this version.
>>>>
>>>> Attached v13 is rebased over master (which includes the streaming read
>>>> API now). I also reset the streaming read object on rescan instead of
>>>> creating a new one each time.
>>>>
>>>> I don't know how much chance any of this has of going in to 17 now, but
>>>> I thought I would start looking into the regression repro Tomas provided
>>>> in [1].
>>>>
>>>
>>> My personal opinion is that we should try to get in as many of the the
>>> refactoring patches as possible, but I think it's probably too late for
>>> the actual switch to the streaming API.
>>
>> Cool. In the attached v15, I have dropped all commits that are related
>> to the streaming read API and included *only* commits that are
>> beneficial to master. A few of the commits are merged or reordered as
>> well.
>>
>> While going through the commits with this new goal in mind (forget about
>> the streaming read API for now), I realized that it doesn't make much
>> sense to just eliminate the layering violation for the current block and
>> leave it there for the prefetch block. I had de-prioritized solving this
>> when I thought we would just delete the prefetch code and replace it
>> with the streaming read.
>>
>> Now that we aren't doing that, I've spent the day trying to resolve the
>> issues with pushing the prefetch code into heapam.c that I cited in [1].
>> 0010 - 0013 are the result of this. They are not very polished yet and
>> need more cleanup and review (especially 0011, which is probably too
>> large), but I am happy with the solution I came up with.
>>
>> Basically, there are too many members needed for bitmap heap scan to put
>> them all in the HeapScanDescData (don't want to bloat it). So, I've made
>> a new BitmapHeapScanDescData and associated begin/rescan/end() functions
>>
>> In the end, with all patches applied, BitmapHeapNext() loops invoking
>> table_scan_bitmap_next_tuple() and table AMs can implement that however
>> they choose.
>>
>>>> I'm also not sure if I should try and group the commits into fewer
>>>> commits now or wait until I have some idea of whether or not the
>>>> approach in 0013 and 0014 is worth pursuing.
>>>>
>>>
>>> You mean whether to pursue the approach in general, or for v17? I think
>>> it looks like the right approach, but for v17 see above :-(
>>>
>>> As for merging, I wouldn't do that. I looked at the commits and while
>>> some of them seem somewhat "trivial", I really like how you organized
>>> the commits, and kept those that just "move" code around, and those that
>>> actually change stuff. It's much easier to understand, IMO.
>>>
>>> I went through the first ~10 commits, and added some review - either as
>>> a separate commit, when possible, in the code as XXX comment, and also
>>> in the commit message. The code tweaks are utterly trivial (whitespace
>>> or indentation to make the line shorter). It shouldn't take much time to
>>> deal with those, I think.
>>
>> Attached v15 incorporates your v14-0002-review.
>>
>> For your v14-0008-review, I actually ended up removing that commit
>> because once I removed everything that was for streaming read API, it
>> became redundant with another commit.
>>
>> For your v14-0010-review, we actually can't easily get rid of those
>> local variables because we make the iterators before we make the scan
>> descriptors and the commit following that commit moves the iterators
>> from the BitmapHeapScanState to the scan descriptor.
>>
>>> I think the main focus should be updating the commit messages. If it was
>>> only a single patch, I'd probably try to write the messages myself, but
>>> with this many patches it'd be great if you could update those and I'll
>>> review that before commit.
>>
>> I did my best to update the commit messages to be less specific and more
>> focused on "why should I care". I found myself wanting to explain why I
>> implemented something the way I did and then getting back into the
>> implementation details again. I'm not sure if I suceeded in having less
>> details and more substance.
>>
>>> I always struggle with writing commit messages myself, and it takes me
>>> ages to write a good one (well, I think the message is good, but who
>>> knows ...). But I think a good message should be concise enough to
>>> explain what and why it's done. It may reference a thread for all the
>>> gory details, but the basic reasoning should be in the commit message.
>>> For example the message for "BitmapPrefetch use prefetch block recheck
>>> for skip fetch" now says that it "makes more sense to do X" but does not
>>> really say why that's the case. The linked message does, but it'd be
>>> good to have that in the message (because how would I know how much of
>>> the thread to read?).
>>
>> I fixed that particular one. I tried to take that feedback and apply it
>> to other commit messages. I don't know how successful I was...
>>
>>> Also, it'd be very helpful if you could update the author & reviewed-by
>>> fields. I'll review those before commit, ofc, but I admit I lost track
>>> of who reviewed which part.
>>
>> I have updated reviewers. I didn't add reviewers on the ones that
>> haven't really been reviewed yet.
>>
>>> I'd focus on the first ~8-9 commits or so for now, we can commit more if
>>> things go reasonably well.
>>
>> Sounds good. I will spend cleanup time on 0010-0013 tomorrow but would
>> love to know if you agree with the direction before I spend more time.
> 
> In attached v16, I've split out 0010-0013 into 0011-0017. I think it is
> much easier to understand.
> 

Damn it, I went through the whole patch series, adding a couple review
comments and tweaks, and was just about to share my version, but you bet
me to it ;-)

Anyway, I've attached it as .tgz in order to not confuse cfbot. All the
review comments are marked with XXX, so grep for that in the patches.
There's two separate patches - the first one suggests a code change, so
it was better to not merge that with your code. The second has just a
couple XXX comments, I'm not sure why I kept it separate.

A couple review comments:

* I think 0001-0009 are 99% ready to. I reworded some of the commit
messages a bit - I realize it's a bit bold, considering you're native
speaker and I'm not. If you could check I didn't make it worse, that
would be great.

* I'm not sure extra_flags is the right way to pass the flag in 0003.
The "extra_" name is a bit weird, and no other table AM functions do it
this way and pass explicit bool flags instead. So my first "review"
commit does it like that. Do you agree it's better that way?

* The one question I'm somewhat unsure about is why Tom chose to use the
"wrong" recheck flag in the 2017 commit, when the correct recheck flag
is readily available. Surely that had a reason, right? But I can't think
of one ...

> While I was doing that, I realized that I should remove the call to
> table_rescan() from ExecReScanBitmapHeapScan() and just rely on the new
> table_rescan_bm() invoked from BitmapHeapNext(). That is done in the
> attached.
> 
> 0010-0018 still need comments updated but I focused on getting the split
> out, reviewable version of them ready. I'll add comments (especially to
> 0011 table AM functions) tomorrow. I also have to double-check if I
> should add any asserts for table AMs about having implemented all of the
> new begin/re/endscan() functions.
> 

I added a couple more comments for those patches (10-12). Chances are
the split in v16 clarifies some of my questions, but it'll have to wait
till the morning ...


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

v15-review.tgz

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

06 April, 16:40:11

On Sat, Apr 06, 2024 at 02:51:45AM +0200, Tomas Vondra wrote:
> 
> 
> On 4/6/24 01:53, Melanie Plageman wrote:
> > On Fri, Apr 05, 2024 at 04:06:34AM -0400, Melanie Plageman wrote:
> >> On Thu, Apr 04, 2024 at 04:35:45PM +0200, Tomas Vondra wrote:
> >>> On 4/4/24 00:57, Melanie Plageman wrote:
> >>>> On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
> >>> I'd focus on the first ~8-9 commits or so for now, we can commit more if
> >>> things go reasonably well.
> >>
> >> Sounds good. I will spend cleanup time on 0010-0013 tomorrow but would
> >> love to know if you agree with the direction before I spend more time.
> > 
> > In attached v16, I've split out 0010-0013 into 0011-0017. I think it is
> > much easier to understand.
> > 
> 
> Anyway, I've attached it as .tgz in order to not confuse cfbot. All the
> review comments are marked with XXX, so grep for that in the patches.
> There's two separate patches - the first one suggests a code change, so
> it was better to not merge that with your code. The second has just a
> couple XXX comments, I'm not sure why I kept it separate.
> 
> A couple review comments:
> 
> * I think 0001-0009 are 99% ready to. I reworded some of the commit
> messages a bit - I realize it's a bit bold, considering you're native
> speaker and I'm not. If you could check I didn't make it worse, that
> would be great.

Attached v17 has *only* patches 0001-0009 with these changes. I will
work on applying the remaining patches, addressing feedback, and adding
comments next.

I have reviewed and incorporated all of your feedback on these patches.
Attached v17 is your exact patches with 1 or 2 *very* slight tweaks to
commit messages (comma splice removal and single word adjustments) as
well as the changes listed below:

I have changed the following:

- 0003 added an assert that rs_empty_tuples_pending is 0 on rescan and
    endscan

- 0004 (your 0005)-- I followed up with Tom, but for now I have just
    removed the XXX and also reworded the message a bit

- 0006 (your 0007) fixed up the variable name (you changed valid ->
    valid_block but it had gotten changed back)

I have open questions on the following:

- 0003: should it be SO_NEED_TUPLES and need_tuples (instead of
    SO_NEED_TUPLE and need_tuple)?

- 0009 (your 0010)
    - Should I mention in the commit message that we added blockno and
        pfblockno in the BitmapHeapScanState only for validation or is that
        too specific?

    - Should I mention that a future (imminent) commit will remove the
        iterators from TableScanDescData and put them in HeapScanDescData? I
        imagine folks don't want those there, but it is easier for the
        progression of commits to put them there first and then move them

    - I'm worried this comment is vague and or potentially not totally
        correct. Should we remove it? I don't think we have conclusive proof
        that this is true.
  /*
   * Adjusting the prefetch iterator before invoking
   * table_scan_bitmap_next_block() keeps prefetch distance higher across
   * the parallel workers.
   */


> * I'm not sure extra_flags is the right way to pass the flag in 0003.
> The "extra_" name is a bit weird, and no other table AM functions do it
> this way and pass explicit bool flags instead. So my first "review"
> commit does it like that. Do you agree it's better that way?

Yes.

> * The one question I'm somewhat unsure about is why Tom chose to use the
> "wrong" recheck flag in the 2017 commit, when the correct recheck flag
> is readily available. Surely that had a reason, right? But I can't think
> of one ...

See above.

> > While I was doing that, I realized that I should remove the call to
> > table_rescan() from ExecReScanBitmapHeapScan() and just rely on the new
> > table_rescan_bm() invoked from BitmapHeapNext(). That is done in the
> > attached.
> > 
> > 0010-0018 still need comments updated but I focused on getting the split
> > out, reviewable version of them ready. I'll add comments (especially to
> > 0011 table AM functions) tomorrow. I also have to double-check if I
> > should add any asserts for table AMs about having implemented all of the
> > new begin/re/endscan() functions.
> > 
> 
> I added a couple more comments for those patches (10-12). Chances are
> the split in v16 clarifies some of my questions, but it'll have to wait
> till the morning ...

Will address this in next mail.

- Melanie

On Sat, Apr 06, 2024 at 04:57:51PM +0200, Tomas Vondra wrote:
> On 4/6/24 15:40, Melanie Plageman wrote:
> > On Sat, Apr 06, 2024 at 02:51:45AM +0200, Tomas Vondra wrote:
> >>
> >>
> >> On 4/6/24 01:53, Melanie Plageman wrote:
> >>> On Fri, Apr 05, 2024 at 04:06:34AM -0400, Melanie Plageman wrote:
> >>>> On Thu, Apr 04, 2024 at 04:35:45PM +0200, Tomas Vondra wrote:
> >>>>> On 4/4/24 00:57, Melanie Plageman wrote:
> >>>>>> On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
> >>>>> I'd focus on the first ~8-9 commits or so for now, we can commit more if
> >>>>> things go reasonably well.
> >>>>
> >>>> Sounds good. I will spend cleanup time on 0010-0013 tomorrow but would
> >>>> love to know if you agree with the direction before I spend more time.
> >>>
> >>> In attached v16, I've split out 0010-0013 into 0011-0017. I think it is
> >>> much easier to understand.
> >>>
> >>
> >> Anyway, I've attached it as .tgz in order to not confuse cfbot. All the
> >> review comments are marked with XXX, so grep for that in the patches.
> >> There's two separate patches - the first one suggests a code change, so
> >> it was better to not merge that with your code. The second has just a
> >> couple XXX comments, I'm not sure why I kept it separate.
> >>
> >> A couple review comments:
> >>
> >> * I think 0001-0009 are 99% ready to. I reworded some of the commit
> >> messages a bit - I realize it's a bit bold, considering you're native
> >> speaker and I'm not. If you could check I didn't make it worse, that
> >> would be great.
> > 
> > Attached v17 has *only* patches 0001-0009 with these changes. I will
> > work on applying the remaining patches, addressing feedback, and adding
> > comments next.
> > 
> > I have reviewed and incorporated all of your feedback on these patches.
> > Attached v17 is your exact patches with 1 or 2 *very* slight tweaks to
> > commit messages (comma splice removal and single word adjustments) as
> > well as the changes listed below:
> > 
> > I have open questions on the following:
> > 
> > - 0003: should it be SO_NEED_TUPLES and need_tuples (instead of
> >     SO_NEED_TUPLE and need_tuple)?
> > 
> 
> I think SO_NEED_TUPLES is more accurate, as we need all tuples from the
> block. But either would work.

Attached v18 changes it to TUPLES/tuples

> 
> > - 0009 (your 0010)
> >     - Should I mention in the commit message that we added blockno and
> >         pfblockno in the BitmapHeapScanState only for validation or is that
> >         too specific?
> > 
> 
> For the commit message I'd say it's too specific, I'd put it in the
> comment before the struct.

It is in the comment for the struct

> >     - I'm worried this comment is vague and or potentially not totally
> >         correct. Should we remove it? I don't think we have conclusive proof
> >         that this is true.
> >   /*
> >    * Adjusting the prefetch iterator before invoking
> >    * table_scan_bitmap_next_block() keeps prefetch distance higher across
> >    * the parallel workers.
> >    */
> > 
> 
> TBH it's not clear to me what "higher across parallel workers" means.
> But it sure shouldn't claim things that we think may not be correct. I
> don't have a good idea how to reword it, though.

I realized it makes more sense to add a FIXME (I used XXX. I'm not when
to use what) with a link to the message where Andres describes why he
thinks it is a bug. If we plan on fixing it, it is good to have a record
of that. And it makes it easier to put a clear and accurate comment.
Done in 0009.

> OK, thanks. If think 0001-0008 are ready to go, with some minor tweaks
> per above (tuple vs. tuples etc.), and the question about the recheck
> flag. If you can do these tweaks, I'll get that committed today and we
> can try to get a couple more patches in tomorrow.

Sounds good.

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

07 April, 00:34:50

On Sat, Apr 06, 2024 at 12:04:23PM -0400, Melanie Plageman wrote:
> On Sat, Apr 06, 2024 at 04:57:51PM +0200, Tomas Vondra wrote:
> > On 4/6/24 15:40, Melanie Plageman wrote:
> > > On Sat, Apr 06, 2024 at 02:51:45AM +0200, Tomas Vondra wrote:
> > >>
> > >>
> > >> On 4/6/24 01:53, Melanie Plageman wrote:
> > >>> On Fri, Apr 05, 2024 at 04:06:34AM -0400, Melanie Plageman wrote:
> > >>>> On Thu, Apr 04, 2024 at 04:35:45PM +0200, Tomas Vondra wrote:
> > >>>>> On 4/4/24 00:57, Melanie Plageman wrote:
> > >>>>>> On Sun, Mar 31, 2024 at 11:45:51AM -0400, Melanie Plageman wrote:
> > >>>>> I'd focus on the first ~8-9 commits or so for now, we can commit more if
> > >>>>> things go reasonably well.
> > >>>>
> > >>>> Sounds good. I will spend cleanup time on 0010-0013 tomorrow but would
> > >>>> love to know if you agree with the direction before I spend more time.
> > >>>
> > >>> In attached v16, I've split out 0010-0013 into 0011-0017. I think it is
> > >>> much easier to understand.
> > >>>
> > >>
> > >> Anyway, I've attached it as .tgz in order to not confuse cfbot. All the
> > >> review comments are marked with XXX, so grep for that in the patches.
> > >> There's two separate patches - the first one suggests a code change, so
> > >> it was better to not merge that with your code. The second has just a
> > >> couple XXX comments, I'm not sure why I kept it separate.
> > >>
> > >> A couple review comments:
> > >>
> > >> * I think 0001-0009 are 99% ready to. I reworded some of the commit
> > >> messages a bit - I realize it's a bit bold, considering you're native
> > >> speaker and I'm not. If you could check I didn't make it worse, that
> > >> would be great.
> > > 
> > > Attached v17 has *only* patches 0001-0009 with these changes. I will
> > > work on applying the remaining patches, addressing feedback, and adding
> > > comments next.
> > > 
> > > I have reviewed and incorporated all of your feedback on these patches.
> > > Attached v17 is your exact patches with 1 or 2 *very* slight tweaks to
> > > commit messages (comma splice removal and single word adjustments) as
> > > well as the changes listed below:
> > > 
> > > I have open questions on the following:
> > > 
> > > - 0003: should it be SO_NEED_TUPLES and need_tuples (instead of
> > >     SO_NEED_TUPLE and need_tuple)?
> > > 
> > 
> > I think SO_NEED_TUPLES is more accurate, as we need all tuples from the
> > block. But either would work.
> 
> Attached v18 changes it to TUPLES/tuples
> 
> > 
> > > - 0009 (your 0010)
> > >     - Should I mention in the commit message that we added blockno and
> > >         pfblockno in the BitmapHeapScanState only for validation or is that
> > >         too specific?
> > > 
> > 
> > For the commit message I'd say it's too specific, I'd put it in the
> > comment before the struct.
> 
> It is in the comment for the struct
> 
> > >     - I'm worried this comment is vague and or potentially not totally
> > >         correct. Should we remove it? I don't think we have conclusive proof
> > >         that this is true.
> > >   /*
> > >    * Adjusting the prefetch iterator before invoking
> > >    * table_scan_bitmap_next_block() keeps prefetch distance higher across
> > >    * the parallel workers.
> > >    */
> > > 
> > 
> > TBH it's not clear to me what "higher across parallel workers" means.
> > But it sure shouldn't claim things that we think may not be correct. I
> > don't have a good idea how to reword it, though.
> 
> I realized it makes more sense to add a FIXME (I used XXX. I'm not when
> to use what) with a link to the message where Andres describes why he
> thinks it is a bug. If we plan on fixing it, it is good to have a record
> of that. And it makes it easier to put a clear and accurate comment.
> Done in 0009.
> 
> > OK, thanks. If think 0001-0008 are ready to go, with some minor tweaks
> > per above (tuple vs. tuples etc.), and the question about the recheck
> > flag. If you can do these tweaks, I'll get that committed today and we
> > can try to get a couple more patches in tomorrow.

Attached v19 rebases the rest of the commits from v17 over the first
nine patches from v18. All patches 0001-0009 are unchanged from v18. I
have made updates and done cleanup on 0010-0021.

- Melanie

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

07 April, 03:27:43

On 4/6/24 23:34, Melanie Plageman wrote:
> ...
>>
>> I realized it makes more sense to add a FIXME (I used XXX. I'm not when
>> to use what) with a link to the message where Andres describes why he
>> thinks it is a bug. If we plan on fixing it, it is good to have a record
>> of that. And it makes it easier to put a clear and accurate comment.
>> Done in 0009.
>>
>>> OK, thanks. If think 0001-0008 are ready to go, with some minor tweaks
>>> per above (tuple vs. tuples etc.), and the question about the recheck
>>> flag. If you can do these tweaks, I'll get that committed today and we
>>> can try to get a couple more patches in tomorrow.
> 
> Attached v19 rebases the rest of the commits from v17 over the first
> nine patches from v18. All patches 0001-0009 are unchanged from v18. I
> have made updates and done cleanup on 0010-0021.
> 

I've pushed 0001-0005, I'll get back to this tomorrow and see how much
more we can get in for v17.

What bothers me on 0006-0008 is that the justification in the commit
messages is "future commit will do something". I think it's fine to have
a separate "prepareatory" patches (I really like how you structured the
patches this way), but it'd be good to have them right before that
"future" commit - I'd like not to have one in v17 and then the "future
commit" in v18, because that's annoying complication for backpatching,
(and probably also when implementing the AM?) etc.

AFAICS for v19, the "future commit" for all three patches (0006-0008) is
0012, which introduces the unified iterator. Is that correct?

Also, for 0008 I'm not sure we could even split it between v17 and v18,
because even if heapam did not use the iterator, what if some other AM
uses it? Without 0012 it'd be a problem for the AM, no?

Would it make sense to move 0009 before these three patches? That seems
like a meaningful change on it's own, right?

FWIW I don't think it's very likely I'll commit the UnifiedTBMIterator
stuff. I do agree with the idea in general, but I think I'd need more
time to think about the details. Sorry about that ...

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

07 April, 07:17:34

On Sun, Apr 07, 2024 at 02:27:43AM +0200, Tomas Vondra wrote:
> On 4/6/24 23:34, Melanie Plageman wrote:
> > ...
> >>
> >> I realized it makes more sense to add a FIXME (I used XXX. I'm not when
> >> to use what) with a link to the message where Andres describes why he
> >> thinks it is a bug. If we plan on fixing it, it is good to have a record
> >> of that. And it makes it easier to put a clear and accurate comment.
> >> Done in 0009.
> >>
> >>> OK, thanks. If think 0001-0008 are ready to go, with some minor tweaks
> >>> per above (tuple vs. tuples etc.), and the question about the recheck
> >>> flag. If you can do these tweaks, I'll get that committed today and we
> >>> can try to get a couple more patches in tomorrow.
> > 
> > Attached v19 rebases the rest of the commits from v17 over the first
> > nine patches from v18. All patches 0001-0009 are unchanged from v18. I
> > have made updates and done cleanup on 0010-0021.
> > 
> 
> I've pushed 0001-0005, I'll get back to this tomorrow and see how much
> more we can get in for v17.

Thanks! I thought about it a bit more, and I got worried about the

    Assert(scan->rs_empty_tuples_pending == 0);

in heap_rescan() and heap_endscan().

I was worried if we don't complete the scan it could end up tripping
incorrectly.

I tried to come up with a query which didn't end up emitting all of the
tuples on the page (using a LIMIT clause), but I struggled to come up
with an example that qualified for the skip fetch optimization and also
returned before completing the scan.

I could work a bit harder tomorrow to try and come up with something.
However, I think it might be safer to just change these to:

    scan->rs_empty_tuples_pending = 0

> What bothers me on 0006-0008 is that the justification in the commit
> messages is "future commit will do something". I think it's fine to have
> a separate "prepareatory" patches (I really like how you structured the
> patches this way), but it'd be good to have them right before that
> "future" commit - I'd like not to have one in v17 and then the "future
> commit" in v18, because that's annoying complication for backpatching,
> (and probably also when implementing the AM?) etc.

Yes, I was thinking about this also.

> AFAICS for v19, the "future commit" for all three patches (0006-0008) is
> 0012, which introduces the unified iterator. Is that correct?

Actually, those patches (v19 0006-0008) were required for v19 0009,
which is why I put them directly before it. 0009 eliminates use of the
TBMIterateResult for control flow in BitmapHeapNext().

I've rephrased the commit messages to not mention future commits and
instead focus on what the changes in the commit are enabling.

v19-0006 actually squashed very easily with v19-0009 and is actually
probably better that way. It is still easy to understand IMO.

In v20, I've attached just the functionality from v19 0006-0009 but in
three patches instead of four.

> Also, for 0008 I'm not sure we could even split it between v17 and v18,
> because even if heapam did not use the iterator, what if some other AM
> uses it? Without 0012 it'd be a problem for the AM, no?

The iterators in the TableScanDescData were introduced in v19-0009. It
is okay for other AMs to use it. In fact, they will want to use it. It
is still initialized and set up in BitmapHeapNext(). They would just
need to call tbm_iterate()/tbm_shared_iterate() on it.

As for how table AMs will cope without the TBMIterateResult passed to
table_scan_bitmap_next_tuple() (which is what v19 0008 did): they can
save the location of the tuples to be scanned somewhere in their scan
descriptor. Heap AM already did this and actually didn't use the
TBMIterateResult at all.

> Would it make sense to move 0009 before these three patches? That seems
> like a meaningful change on it's own, right?

Since v19 0009 requires these patches, I don't think we could do that.
I think up to and including 0009 would be an improvement in clarity and
function.

As for all the patches after 0009, I've dropped them from this version.
We are out of time, and they need more thought.

After we decided not to pursue streaming bitmapheapscan for 17, I wanted
to make sure we removed the prefetch code table AM violation -- since we
weren't deleting that code. So what started out as me looking for a way
to clean up one commit ended up becoming a much larger project. Sorry
about that last minute code explosion! I do think there is a way to do
it right and make it nice. Also that violation would be gone if we
figure out how to get streaming bitmapheapscan behaving correctly.

So, there's just more motivation to make streaming bitmapheapscan
awesome for 18!

Given all that, I've only included the three patches I think we are
considering (former v19 0006-0008). They are largely the same as you saw
them last except for squashing the two commits I mentioned above and
updating all of the commit messages.

> FWIW I don't think it's very likely I'll commit the UnifiedTBMIterator
> stuff. I do agree with the idea in general, but I think I'd need more
> time to think about the details. Sorry about that ...

Yes, that makes total sense. I 100% agree.

I do think the UnifiedTBMIterator (maybe the name is not good, though)
is a good way to simplify the BitmapHeapScan code and is applicable to
any future TIDBitmap user with both a parallel and serial
implementation. So, there's a nice, small patch I can register for July.

Thanks again for taking time to work on this!

- Melanie

On Sun, Apr 7, 2024 at 10:42 PM Tomas Vondra <tomas.vondra@enterprisedb.com> wrote:

create table t (a int, b int) with (fillfactor=10);
insert into t select mod((i/22),2), (i/22) from generate_series(0,1000)
S(i);
create index on t(a);
vacuum analyze t;

set enable_indexonlyscan = off;
set enable_seqscan = off;
explain (analyze, verbose) select 1 from (values (1)) s(x) where exists
(select * from t where a = x);

KABOOM!

FWIW, it seems to me that this assert could be triggered in cases where,
during a join, not all inner tuples need to be scanned before skipping to
next outer tuple. This can happen for 'single_match' or anti-join.

The query provided by Tomas is an example of 'single_match' case. Here
is a query for anti-join that can also trigger this assert.

explain (analyze, verbose)
select t1.a from t t1 left join t t2 on t2.a = 1 where t2.a is null;
server closed the connection unexpectedly

Thanks
Richard

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Michael Paquier

Date:

18 April, 10:10:29

On Sun, Apr 07, 2024 at 10:54:56AM -0400, Melanie Plageman wrote:
> I've added an open item [1], because what's one open item when you can
> have two? (me)

And this is still an open item as of today.  What's the plan to move
forward here?
--
Michael

Attachment

signature.asc

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

18 April, 12:39:22

On 4/18/24 09:10, Michael Paquier wrote:
> On Sun, Apr 07, 2024 at 10:54:56AM -0400, Melanie Plageman wrote:
>> I've added an open item [1], because what's one open item when you can
>> have two? (me)
> 
> And this is still an open item as of today.  What's the plan to move
> forward here?

AFAIK the plan is to replace the asserts with actually resetting the
rs_empty_tuples_pending field to 0, as suggested by Melanie a week ago.
I assume she was busy with the post-freeze AM reworks last week, so this
was on a back burner.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

22 April, 20:01:51

On Thu, Apr 18, 2024 at 5:39 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 4/18/24 09:10, Michael Paquier wrote:
> > On Sun, Apr 07, 2024 at 10:54:56AM -0400, Melanie Plageman wrote:
> >> I've added an open item [1], because what's one open item when you can
> >> have two? (me)
> >
> > And this is still an open item as of today.  What's the plan to move
> > forward here?
>
> AFAIK the plan is to replace the asserts with actually resetting the
> rs_empty_tuples_pending field to 0, as suggested by Melanie a week ago.
> I assume she was busy with the post-freeze AM reworks last week, so this
> was on a back burner.

yep, sorry. Also I took a few days off. I'm just catching up today. I
want to pop in one of Richard or Tomas' examples as a test, since it
seems like it would add some coverage. I will have a patch soon.

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

23 April, 19:05:05

On Mon, Apr 22, 2024 at 1:01 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Thu, Apr 18, 2024 at 5:39 AM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
> >
> > On 4/18/24 09:10, Michael Paquier wrote:
> > > On Sun, Apr 07, 2024 at 10:54:56AM -0400, Melanie Plageman wrote:
> > >> I've added an open item [1], because what's one open item when you can
> > >> have two? (me)
> > >
> > > And this is still an open item as of today.  What's the plan to move
> > > forward here?
> >
> > AFAIK the plan is to replace the asserts with actually resetting the
> > rs_empty_tuples_pending field to 0, as suggested by Melanie a week ago.
> > I assume she was busy with the post-freeze AM reworks last week, so this
> > was on a back burner.
>
> yep, sorry. Also I took a few days off. I'm just catching up today. I
> want to pop in one of Richard or Tomas' examples as a test, since it
> seems like it would add some coverage. I will have a patch soon.

The patch with a fix is attached. I put the test in
src/test/regress/sql/join.sql. It isn't the perfect location because
it is testing something exercisable with a join but not directly
related to the fact that it is a join. I also considered
src/test/regress/sql/select.sql, but it also isn't directly related to
the query being a SELECT query. If there is a better place for a test
of a bitmap heap scan edge case, let me know.

One other note: there is some concurrency effect in the parallel
schedule group containing "join" where you won't trip the assert if
all the tests in that group in the parallel schedule are run. But, if
you would like to verify that the test exercises the correct code,
just reduce the group containing "join".

- Melanie

Attachment

v1-0001-BitmapHeapScan-Remove-incorrect-assert.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

24 April, 01:43:38


On 4/23/24 18:05, Melanie Plageman wrote:
> On Mon, Apr 22, 2024 at 1:01 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
>>
>> On Thu, Apr 18, 2024 at 5:39 AM Tomas Vondra
>> <tomas.vondra@enterprisedb.com> wrote:
>>>
>>> On 4/18/24 09:10, Michael Paquier wrote:
>>>> On Sun, Apr 07, 2024 at 10:54:56AM -0400, Melanie Plageman wrote:
>>>>> I've added an open item [1], because what's one open item when you can
>>>>> have two? (me)
>>>>
>>>> And this is still an open item as of today.  What's the plan to move
>>>> forward here?
>>>
>>> AFAIK the plan is to replace the asserts with actually resetting the
>>> rs_empty_tuples_pending field to 0, as suggested by Melanie a week ago.
>>> I assume she was busy with the post-freeze AM reworks last week, so this
>>> was on a back burner.
>>
>> yep, sorry. Also I took a few days off. I'm just catching up today. I
>> want to pop in one of Richard or Tomas' examples as a test, since it
>> seems like it would add some coverage. I will have a patch soon.
> 
> The patch with a fix is attached. I put the test in
> src/test/regress/sql/join.sql. It isn't the perfect location because
> it is testing something exercisable with a join but not directly
> related to the fact that it is a join. I also considered
> src/test/regress/sql/select.sql, but it also isn't directly related to
> the query being a SELECT query. If there is a better place for a test
> of a bitmap heap scan edge case, let me know.
> 

I don't see a problem with adding this to join.sql - why wouldn't this
count as something related to a join? Sure, it's not like this code
matters only for joins, but if you look at join.sql that applies to a
number of other tests (e.g. there are a couple btree tests).

That being said, it'd be good to explain in the comment why we're
testing this particular plan, not just what the plan looks like.

> One other note: there is some concurrency effect in the parallel
> schedule group containing "join" where you won't trip the assert if
> all the tests in that group in the parallel schedule are run. But, if
> you would like to verify that the test exercises the correct code,
> just reduce the group containing "join".
> 

That is ... interesting. Doesn't that mean that most test runs won't
actually detect the problem? That would make the test a bit useless.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

24 April, 23:46:19

On Tue, Apr 23, 2024 at 6:43 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 4/23/24 18:05, Melanie Plageman wrote:
> > The patch with a fix is attached. I put the test in
> > src/test/regress/sql/join.sql. It isn't the perfect location because
> > it is testing something exercisable with a join but not directly
> > related to the fact that it is a join. I also considered
> > src/test/regress/sql/select.sql, but it also isn't directly related to
> > the query being a SELECT query. If there is a better place for a test
> > of a bitmap heap scan edge case, let me know.
>
> I don't see a problem with adding this to join.sql - why wouldn't this
> count as something related to a join? Sure, it's not like this code
> matters only for joins, but if you look at join.sql that applies to a
> number of other tests (e.g. there are a couple btree tests).

I suppose it's true that other tests in this file use joins to test
other code. I guess if we limited join.sql to containing tests of join
implementation, it would be rather small. I just imagined it would be
nice if tests were grouped by what they were testing -- not how they
were testing it.

> That being said, it'd be good to explain in the comment why we're
> testing this particular plan, not just what the plan looks like.

You mean I should explain in the test comment why I included the
EXPLAIN plan output? (because it needs to contain a bitmapheapscan to
actually be testing anything)

I do have a detailed explanation in my test comment of why this
particular query exercises the code we want to test.

> > One other note: there is some concurrency effect in the parallel
> > schedule group containing "join" where you won't trip the assert if
> > all the tests in that group in the parallel schedule are run. But, if
> > you would like to verify that the test exercises the correct code,
> > just reduce the group containing "join".
> >
>
> That is ... interesting. Doesn't that mean that most test runs won't
> actually detect the problem? That would make the test a bit useless.

Yes, I should really have thought about it more. After further
investigation, I found that the reason it doesn't trip the assert when
the join test is run concurrently with other tests is that the SELECT
query doesn't use the skip fetch optimization because the VACUUM
doesn't set the pages all visible in the VM. In this case, it's
because the tuples' xmins are not before VacuumCutoffs->OldestXmin
(which is derived from GetOldestNonRemovableTransactionId()).

After thinking about it more, I suppose we can't add a test that
relies on the relation being all visible in the VM in a group in the
parallel schedule. I'm not sure this edge case is important enough to
merit its own group or an isolation test. What do you think?

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

26 April, 02:03:57

On Wed, Apr 24, 2024 at 4:46 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Tue, Apr 23, 2024 at 6:43 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
> >
> > On 4/23/24 18:05, Melanie Plageman wrote:
> > > One other note: there is some concurrency effect in the parallel
> > > schedule group containing "join" where you won't trip the assert if
> > > all the tests in that group in the parallel schedule are run. But, if
> > > you would like to verify that the test exercises the correct code,
> > > just reduce the group containing "join".
> > >
> >
> > That is ... interesting. Doesn't that mean that most test runs won't
> > actually detect the problem? That would make the test a bit useless.
>
> Yes, I should really have thought about it more. After further
> investigation, I found that the reason it doesn't trip the assert when
> the join test is run concurrently with other tests is that the SELECT
> query doesn't use the skip fetch optimization because the VACUUM
> doesn't set the pages all visible in the VM. In this case, it's
> because the tuples' xmins are not before VacuumCutoffs->OldestXmin
> (which is derived from GetOldestNonRemovableTransactionId()).
>
> After thinking about it more, I suppose we can't add a test that
> relies on the relation being all visible in the VM in a group in the
> parallel schedule. I'm not sure this edge case is important enough to
> merit its own group or an isolation test. What do you think?

Andres rightly pointed out to me off-list that if I just used a temp
table, the table would only be visible to the testing backend anyway.
I've done that in the attached v2. Now the test is deterministic.

- Melanie

Attachment

v2-0001-BitmapHeapScan-Remove-incorrect-assert.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tom Lane

Date:

26 April, 02:28:45

Melanie Plageman <melanieplageman@gmail.com> writes:
> On Wed, Apr 24, 2024 at 4:46 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
>> After thinking about it more, I suppose we can't add a test that
>> relies on the relation being all visible in the VM in a group in the
>> parallel schedule. I'm not sure this edge case is important enough to
>> merit its own group or an isolation test. What do you think?

> Andres rightly pointed out to me off-list that if I just used a temp
> table, the table would only be visible to the testing backend anyway.
> I've done that in the attached v2. Now the test is deterministic.

Hmm, is that actually true?  There's no more reason to think a tuple
in a temp table is old enough to be visible to all other sessions
than one in any other table.  It could be all right if we had a
special-case rule for setting all-visible in temp tables.  Which
indeed I thought we had, but I can't find any evidence of that in
vacuumlazy.c, nor did a trawl of the commit log turn up anything
promising.  Am I just looking in the wrong place?

            regards, tom lane

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tom Lane

Date:

26 April, 02:57:19

I wrote:
> Hmm, is that actually true?  There's no more reason to think a tuple
> in a temp table is old enough to be visible to all other sessions
> than one in any other table.  It could be all right if we had a
> special-case rule for setting all-visible in temp tables.  Which
> indeed I thought we had, but I can't find any evidence of that in
> vacuumlazy.c, nor did a trawl of the commit log turn up anything
> promising.  Am I just looking in the wrong place?

Ah, never mind that --- I must be looking in the wrong place.
Direct experimentation proves that VACUUM will set all-visible bits
for temp tables even in the presence of concurrent transactions.

            regards, tom lane

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

26 April, 16:04:22

On Thu, Apr 25, 2024 at 7:57 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> I wrote:
> > Hmm, is that actually true?  There's no more reason to think a tuple
> > in a temp table is old enough to be visible to all other sessions
> > than one in any other table.  It could be all right if we had a
> > special-case rule for setting all-visible in temp tables.  Which
> > indeed I thought we had, but I can't find any evidence of that in
> > vacuumlazy.c, nor did a trawl of the commit log turn up anything
> > promising.  Am I just looking in the wrong place?
>
> Ah, never mind that --- I must be looking in the wrong place.
> Direct experimentation proves that VACUUM will set all-visible bits
> for temp tables even in the presence of concurrent transactions.

If this seems correct to you, are you okay with the rest of the fix
and test? We could close this open item once the patch is acceptable.

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Daniel Gustafsson

Date:

30 April, 15:07:43

> On 26 Apr 2024, at 15:04, Melanie Plageman <melanieplageman@gmail.com> wrote:

> If this seems correct to you, are you okay with the rest of the fix
> and test? We could close this open item once the patch is acceptable.

From reading the discussion and the patch this seems like the right fix to me.
Does the test added here aptly cover 04e72ed617be in terms its functionality?

--
Daniel Gustafsson

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

03 May, 00:31:00

On 4/30/24 14:07, Daniel Gustafsson wrote:
>> On 26 Apr 2024, at 15:04, Melanie Plageman <melanieplageman@gmail.com> wrote:
> 
>> If this seems correct to you, are you okay with the rest of the fix
>> and test? We could close this open item once the patch is acceptable.
> 
> From reading the discussion and the patch this seems like the right fix to me.

I agree.

> Does the test added here aptly cover 04e72ed617be in terms its functionality?
> 

AFAIK the test fails without the fix and works with it, so I believe it
does cover the relevant functionality.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

03 May, 00:37:06


On 4/24/24 22:46, Melanie Plageman wrote:
> On Tue, Apr 23, 2024 at 6:43 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 4/23/24 18:05, Melanie Plageman wrote:
>>> The patch with a fix is attached. I put the test in
>>> src/test/regress/sql/join.sql. It isn't the perfect location because
>>> it is testing something exercisable with a join but not directly
>>> related to the fact that it is a join. I also considered
>>> src/test/regress/sql/select.sql, but it also isn't directly related to
>>> the query being a SELECT query. If there is a better place for a test
>>> of a bitmap heap scan edge case, let me know.
>>
>> I don't see a problem with adding this to join.sql - why wouldn't this
>> count as something related to a join? Sure, it's not like this code
>> matters only for joins, but if you look at join.sql that applies to a
>> number of other tests (e.g. there are a couple btree tests).
> 
> I suppose it's true that other tests in this file use joins to test
> other code. I guess if we limited join.sql to containing tests of join
> implementation, it would be rather small. I just imagined it would be
> nice if tests were grouped by what they were testing -- not how they
> were testing it.
> 
>> That being said, it'd be good to explain in the comment why we're
>> testing this particular plan, not just what the plan looks like.
> 
> You mean I should explain in the test comment why I included the
> EXPLAIN plan output? (because it needs to contain a bitmapheapscan to
> actually be testing anything)
> 

No, I meant that the comment before the test describes a couple
requirements the plan needs to meet (no need to process all inner
tuples, bitmapscan eligible for skip_fetch on outer side, ...), but it
does not explain why we're testing that plan.

I could get to that by doing git-blame to see what commit added this
code, and then read the linked discussion. Perhaps that's enough, but
maybe the comment could say something like "verify we properly discard
tuples on rescans" or something like that?


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

10 May, 22:48:24

On Thu, May 2, 2024 at 5:37 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
>
>
> On 4/24/24 22:46, Melanie Plageman wrote:
> > On Tue, Apr 23, 2024 at 6:43 PM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >>
> >> On 4/23/24 18:05, Melanie Plageman wrote:
> >>> The patch with a fix is attached. I put the test in
> >>> src/test/regress/sql/join.sql. It isn't the perfect location because
> >>> it is testing something exercisable with a join but not directly
> >>> related to the fact that it is a join. I also considered
> >>> src/test/regress/sql/select.sql, but it also isn't directly related to
> >>> the query being a SELECT query. If there is a better place for a test
> >>> of a bitmap heap scan edge case, let me know.
> >>
> >> I don't see a problem with adding this to join.sql - why wouldn't this
> >> count as something related to a join? Sure, it's not like this code
> >> matters only for joins, but if you look at join.sql that applies to a
> >> number of other tests (e.g. there are a couple btree tests).
> >
> > I suppose it's true that other tests in this file use joins to test
> > other code. I guess if we limited join.sql to containing tests of join
> > implementation, it would be rather small. I just imagined it would be
> > nice if tests were grouped by what they were testing -- not how they
> > were testing it.
> >
> >> That being said, it'd be good to explain in the comment why we're
> >> testing this particular plan, not just what the plan looks like.
> >
> > You mean I should explain in the test comment why I included the
> > EXPLAIN plan output? (because it needs to contain a bitmapheapscan to
> > actually be testing anything)
> >
>
> No, I meant that the comment before the test describes a couple
> requirements the plan needs to meet (no need to process all inner
> tuples, bitmapscan eligible for skip_fetch on outer side, ...), but it
> does not explain why we're testing that plan.
>
> I could get to that by doing git-blame to see what commit added this
> code, and then read the linked discussion. Perhaps that's enough, but
> maybe the comment could say something like "verify we properly discard
> tuples on rescans" or something like that?

Attached is v3. I didn't use your exact language because the test
wouldn't actually verify that we properly discard the tuples. Whether
or not the empty tuples are all emitted, it just resets the counter to
0. I decided to go with "exercise" instead.

- Melanie

Attachment

v3-0001-BitmapHeapScan-Remove-incorrect-assert.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

11 May, 22:18:21

On 5/10/24 21:48, Melanie Plageman wrote:
> On Thu, May 2, 2024 at 5:37 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>>
>>
>> On 4/24/24 22:46, Melanie Plageman wrote:
>>> On Tue, Apr 23, 2024 at 6:43 PM Tomas Vondra
>>> <tomas.vondra@enterprisedb.com> wrote:
>>>>
>>>> On 4/23/24 18:05, Melanie Plageman wrote:
>>>>> The patch with a fix is attached. I put the test in
>>>>> src/test/regress/sql/join.sql. It isn't the perfect location because
>>>>> it is testing something exercisable with a join but not directly
>>>>> related to the fact that it is a join. I also considered
>>>>> src/test/regress/sql/select.sql, but it also isn't directly related to
>>>>> the query being a SELECT query. If there is a better place for a test
>>>>> of a bitmap heap scan edge case, let me know.
>>>>
>>>> I don't see a problem with adding this to join.sql - why wouldn't this
>>>> count as something related to a join? Sure, it's not like this code
>>>> matters only for joins, but if you look at join.sql that applies to a
>>>> number of other tests (e.g. there are a couple btree tests).
>>>
>>> I suppose it's true that other tests in this file use joins to test
>>> other code. I guess if we limited join.sql to containing tests of join
>>> implementation, it would be rather small. I just imagined it would be
>>> nice if tests were grouped by what they were testing -- not how they
>>> were testing it.
>>>
>>>> That being said, it'd be good to explain in the comment why we're
>>>> testing this particular plan, not just what the plan looks like.
>>>
>>> You mean I should explain in the test comment why I included the
>>> EXPLAIN plan output? (because it needs to contain a bitmapheapscan to
>>> actually be testing anything)
>>>
>>
>> No, I meant that the comment before the test describes a couple
>> requirements the plan needs to meet (no need to process all inner
>> tuples, bitmapscan eligible for skip_fetch on outer side, ...), but it
>> does not explain why we're testing that plan.
>>
>> I could get to that by doing git-blame to see what commit added this
>> code, and then read the linked discussion. Perhaps that's enough, but
>> maybe the comment could say something like "verify we properly discard
>> tuples on rescans" or something like that?
> 
> Attached is v3. I didn't use your exact language because the test
> wouldn't actually verify that we properly discard the tuples. Whether
> or not the empty tuples are all emitted, it just resets the counter to
> 0. I decided to go with "exercise" instead.
> 

I did go over the v3 patch, did a bunch of tests, and I think it's fine
and ready to go. The one thing that might need some minor tweaks is the
commit message.

1) Isn't the subject "Remove incorrect assert" a bit misleading, as the
patch does not simply remove an assert, but replaces it with a reset of
the field the assert used to check? (The commit message does not mention
this either, at least not explicitly.)

2) The "heap AM-specific bitmap heap scan code" sounds a bit strange to
me, isn't the first "heap" unnecessary?


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

13 May, 17:05:03

On Sat, May 11, 2024 at 3:18 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 5/10/24 21:48, Melanie Plageman wrote:
> > Attached is v3. I didn't use your exact language because the test
> > wouldn't actually verify that we properly discard the tuples. Whether
> > or not the empty tuples are all emitted, it just resets the counter to
> > 0. I decided to go with "exercise" instead.
> >
>
> I did go over the v3 patch, did a bunch of tests, and I think it's fine
> and ready to go. The one thing that might need some minor tweaks is the
> commit message.
>
> 1) Isn't the subject "Remove incorrect assert" a bit misleading, as the
> patch does not simply remove an assert, but replaces it with a reset of
> the field the assert used to check? (The commit message does not mention
> this either, at least not explicitly.)

I've updated the commit message.

> 2) The "heap AM-specific bitmap heap scan code" sounds a bit strange to
> me, isn't the first "heap" unnecessary?

bitmap heap scan has been used to refer to bitmap table scans, as the
name wasn't changed from heap when the table AM API was added (e.g.
BitmapHeapNext() is used by all table AMs doing bitmap table scans).
04e72ed617be specifically pushed the skip fetch optimization into heap
implementations of bitmap table scan callbacks, so it was important to
make this distinction. I've changed the commit message to say heap AM
implementations of bitmap table scan callbacks.

While looking at the patch again, I wondered if I should set
enable_material=false in the test. It doesn't matter from the
perspective of exercising the correct code; however, I wasn't sure if
disabling materialization would make the test more resilient against
future planner changes which could cause it to incorrectly fail.

- Melanie

Attachment

v4-0001-BitmapHeapScan-Replace-incorrect-assert-with-rein.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Michael Paquier

Date:

14 May, 09:18:16

On Mon, May 13, 2024 at 10:05:03AM -0400, Melanie Plageman wrote:
> Remove the assert and reset the field on which it previously asserted to
> avoid incorrectly emitting NULL-filled tuples from a previous scan on
> rescan.

> -    Assert(scan->rs_empty_tuples_pending == 0);
> +    scan->rs_empty_tuples_pending = 0;

Perhaps this should document the reason why the reset is done in these
two paths rather than let the reader guess it?  And this is about
avoiding emitting some tuples from a previous scan.

> +SET enable_indexonlyscan = off;
> +set enable_indexscan = off;
> +SET enable_seqscan = off;

Nit: adjusting the casing of the second SET here.
--
Michael

Attachment

signature.asc

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

14 May, 20:42:09

On Tue, May 14, 2024 at 2:18 AM Michael Paquier <michael@paquier.xyz> wrote:
>
> On Mon, May 13, 2024 at 10:05:03AM -0400, Melanie Plageman wrote:
> > Remove the assert and reset the field on which it previously asserted to
> > avoid incorrectly emitting NULL-filled tuples from a previous scan on
> > rescan.
>
> > -     Assert(scan->rs_empty_tuples_pending == 0);
> > +     scan->rs_empty_tuples_pending = 0;
>
> Perhaps this should document the reason why the reset is done in these
> two paths rather than let the reader guess it?  And this is about
> avoiding emitting some tuples from a previous scan.

I've added a comment to heap_rescan() in the attached v5. Doing so
made me realize that we shouldn't bother resetting it in
heap_endscan(). Doing so is perhaps more confusing, because it implies
that field may somehow be used later. I've removed the reset of
rs_empty_tuples_pending from heap_endscan().

> > +SET enable_indexonlyscan = off;
> > +set enable_indexscan = off;
> > +SET enable_seqscan = off;
>
> Nit: adjusting the casing of the second SET here.

I've fixed this. I've also set enable_material off as I mentioned I
might in my earlier mail.

- Melanie

Attachment

v5-0001-BitmapHeapScan-Remove-incorrect-assert-and-reset-.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

14 May, 21:33:32

On 5/14/24 19:42, Melanie Plageman wrote:
> On Tue, May 14, 2024 at 2:18 AM Michael Paquier <michael@paquier.xyz> wrote:
>>
>> On Mon, May 13, 2024 at 10:05:03AM -0400, Melanie Plageman wrote:
>>> Remove the assert and reset the field on which it previously asserted to
>>> avoid incorrectly emitting NULL-filled tuples from a previous scan on
>>> rescan.
>>
>>> -     Assert(scan->rs_empty_tuples_pending == 0);
>>> +     scan->rs_empty_tuples_pending = 0;
>>
>> Perhaps this should document the reason why the reset is done in these
>> two paths rather than let the reader guess it?  And this is about
>> avoiding emitting some tuples from a previous scan.
> 
> I've added a comment to heap_rescan() in the attached v5. Doing so
> made me realize that we shouldn't bother resetting it in
> heap_endscan(). Doing so is perhaps more confusing, because it implies
> that field may somehow be used later. I've removed the reset of
> rs_empty_tuples_pending from heap_endscan().
> 

+1

>>> +SET enable_indexonlyscan = off;
>>> +set enable_indexscan = off;
>>> +SET enable_seqscan = off;
>>
>> Nit: adjusting the casing of the second SET here.
> 
> I've fixed this. I've also set enable_material off as I mentioned I
> might in my earlier mail.
> 

I'm not sure this (setting more and more GUCs to prevent hypothetical
plan changes) is a good practice. Because how do you know the plan does
not change for some other unexpected reason, possibly in the future?

IMHO if the test requires a specific plan, it's better to do an actual
"explain (rows off, costs off)" to check that.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

14 May, 21:40:14

On Tue, May 14, 2024 at 2:33 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 5/14/24 19:42, Melanie Plageman wrote:
> > I've fixed this. I've also set enable_material off as I mentioned I
> > might in my earlier mail.
> >
>
> I'm not sure this (setting more and more GUCs to prevent hypothetical
> plan changes) is a good practice. Because how do you know the plan does
> not change for some other unexpected reason, possibly in the future?

Sure. So, you think it is better not to have enable_material = false?

> IMHO if the test requires a specific plan, it's better to do an actual
> "explain (rows off, costs off)" to check that.

When you say "rows off", do you mean do something to ensure that it
doesn't return tuples? Because I don't see a ROWS option for EXPLAIN
in the docs.

- Melanie

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

14 May, 21:44:45

On 5/14/24 20:40, Melanie Plageman wrote:
> On Tue, May 14, 2024 at 2:33 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 5/14/24 19:42, Melanie Plageman wrote:
>>> I've fixed this. I've also set enable_material off as I mentioned I
>>> might in my earlier mail.
>>>
>>
>> I'm not sure this (setting more and more GUCs to prevent hypothetical
>> plan changes) is a good practice. Because how do you know the plan does
>> not change for some other unexpected reason, possibly in the future?
> 
> Sure. So, you think it is better not to have enable_material = false?
> 

Right. Unless it's actually needed to force the necessary plan.

>> IMHO if the test requires a specific plan, it's better to do an actual
>> "explain (rows off, costs off)" to check that.
> 
> When you say "rows off", do you mean do something to ensure that it
> doesn't return tuples? Because I don't see a ROWS option for EXPLAIN
> in the docs.
> 

Sorry, I meant to hide the cardinality estimates in the explain, but I
got confused. "COSTS OFF" is enough.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

14 May, 22:11:39

On Tue, May 14, 2024 at 2:44 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 5/14/24 20:40, Melanie Plageman wrote:
> > On Tue, May 14, 2024 at 2:33 PM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >>
> >> On 5/14/24 19:42, Melanie Plageman wrote:
> >>> I've fixed this. I've also set enable_material off as I mentioned I
> >>> might in my earlier mail.
> >>>
> >>
> >> I'm not sure this (setting more and more GUCs to prevent hypothetical
> >> plan changes) is a good practice. Because how do you know the plan does
> >> not change for some other unexpected reason, possibly in the future?
> >
> > Sure. So, you think it is better not to have enable_material = false?
> >
>
> Right. Unless it's actually needed to force the necessary plan.

Attached v6 does not use enable_material = false (as it is not needed).

- Melanie

Attachment

v6-0001-BitmapHeapScan-Remove-incorrect-assert-and-reset-.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Alvaro Herrera

Date:

14 May, 23:05:35

On 2024-May-14, Tomas Vondra wrote:

> On 5/14/24 19:42, Melanie Plageman wrote:
> 
> >>> +SET enable_indexonlyscan = off;
> >>> +set enable_indexscan = off;
> >>> +SET enable_seqscan = off;
> >>
> >> Nit: adjusting the casing of the second SET here.
> > 
> > I've fixed this. I've also set enable_material off as I mentioned I
> > might in my earlier mail.
>
> I'm not sure this (setting more and more GUCs to prevent hypothetical
> plan changes) is a good practice. Because how do you know the plan does
> not change for some other unexpected reason, possibly in the future?

I wonder why it resets enable_indexscan at all.  I see that this query
first tries a seqscan, then if you disable that it tries an index only
scan, and if you disable that you get the expected bitmap indexscan.
But an indexscan doesn't seem to be in the cards.

> IMHO if the test requires a specific plan, it's better to do an actual
> "explain (rows off, costs off)" to check that.

That's already in the patch, right?

I do wonder how do we _know_ that the test is testing what it wants to
test:
                       QUERY PLAN                        
─────────────────────────────────────────────────────────
 Nested Loop Anti Join
   ->  Seq Scan on skip_fetch t1
   ->  Materialize
         ->  Bitmap Heap Scan on skip_fetch t2
               Recheck Cond: (a = 1)
               ->  Bitmap Index Scan on skip_fetch_a_idx
                     Index Cond: (a = 1)

Is it because of the shape of the index condition?  Maybe it's worth
explaining in the comments for the tests.

BTW, I was running the explain while desultorily enabling and disabling
these GUCs and hit this assertion failure:

#4  0x000055e6c72afe28 in ExceptionalCondition (conditionName=conditionName@entry=0x55e6c731a928
"scan->rs_empty_tuples_pending== 0", 
 
    fileName=fileName@entry=0x55e6c731a3b0
"../../../../../../../../../pgsql/source/master/src/backend/access/heap/heapam.c",lineNumber=lineNumber@entry=1219)
 
    at ../../../../../../../../../pgsql/source/master/src/backend/utils/error/assert.c:66
#5  0x000055e6c6e2e0c7 in heap_endscan (sscan=0x55e6c7b63e28) at
../../../../../../../../../pgsql/source/master/src/backend/access/heap/heapam.c:1219
#6  0x000055e6c6fb35a7 in ExecEndPlan (estate=0x55e6c7a7e9d0, planstate=<optimized out>) at
../../../../../../../../pgsql/source/master/src/backend/executor/execMain.c:1485
#7  standard_ExecutorEnd (queryDesc=0x55e6c7a736b8) at
../../../../../../../../pgsql/source/master/src/backend/executor/execMain.c:501
#8  0x000055e6c6f4d9aa in ExplainOnePlan (plannedstmt=plannedstmt@entry=0x55e6c7a735a8, into=into@entry=0x0,
es=es@entry=0x55e6c7a448b8,
 
    queryString=queryString@entry=0x55e6c796c210 "EXPLAIN (analyze, verbose, COSTS OFF) SELECT t1.a FROM skip_fetch t1
LEFTJOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS NULL;", params=params@entry=0x0, 
 
    queryEnv=queryEnv@entry=0x0, planduration=0x7ffe8a291848, bufusage=0x0, mem_counters=0x0) at
../../../../../../../../pgsql/source/master/src/backend/commands/explain.c:770
#9  0x000055e6c6f4e257 in standard_ExplainOneQuery (query=<optimized out>, cursorOptions=2048, into=0x0,
es=0x55e6c7a448b8,
 
    queryString=0x55e6c796c210 "EXPLAIN (analyze, verbose, COSTS OFF) SELECT t1.a FROM skip_fetch t1 LEFT JOIN
skip_fetcht2 ON t2.a = 1 WHERE t2.a IS NULL;", params=0x0, queryEnv=0x0)
 
    at ../../../../../../../../pgsql/source/master/src/backend/commands/explain.c:502

I couldn't reproduce it again, though -- and for sure I don't know what
it means.  All three GUCs are set false in the core.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"Here's a general engineering tip: if the non-fun part is too complex for you
to figure out, that might indicate the fun part is too ambitious." (John Naylor)
https://postgr.es/m/CAFBsxsG4OWHBbSDM%3DsSeXrQGOtkPiOEOuME4yD7Ce41NtaAD9g%40mail.gmail.com

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Alvaro Herrera

Date:

14 May, 23:09:39

On 2024-May-14, Alvaro Herrera wrote:


> BTW, I was running the explain while desultorily enabling and disabling
> these GUCs and hit this assertion failure:
> 
> #4  0x000055e6c72afe28 in ExceptionalCondition (conditionName=conditionName@entry=0x55e6c731a928
"scan->rs_empty_tuples_pending== 0", 
 
>     fileName=fileName@entry=0x55e6c731a3b0
"../../../../../../../../../pgsql/source/master/src/backend/access/heap/heapam.c",lineNumber=lineNumber@entry=1219)
 
>     at ../../../../../../../../../pgsql/source/master/src/backend/utils/error/assert.c:66

Ah, I see now that this is precisely the assertion that this patch
removes.  Nevermind ...

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"This is what I like so much about PostgreSQL.  Most of the surprises
are of the "oh wow!  That's cool" Not the "oh shit!" kind.  :)"
Scott Marlowe, http://archives.postgresql.org/pgsql-admin/2008-10/msg00152.php

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

15 May, 00:19:29

On Tue, May 14, 2024 at 4:05 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> On 2024-May-14, Tomas Vondra wrote:
>
> I wonder why it resets enable_indexscan at all.  I see that this query
> first tries a seqscan, then if you disable that it tries an index only
> scan, and if you disable that you get the expected bitmap indexscan.
> But an indexscan doesn't seem to be in the cards.

Ah, yes. That is true. I think I added that when, in an older version
of the test, I had a query that did try an index scan before bitmap
heap scan. I've removed that guc from the attached v7.

> > IMHO if the test requires a specific plan, it's better to do an actual
> > "explain (rows off, costs off)" to check that.
>
> That's already in the patch, right?

Yep.

> I do wonder how do we _know_ that the test is testing what it wants to
> test:
>                        QUERY PLAN
> ─────────────────────────────────────────────────────────
>  Nested Loop Anti Join
>    ->  Seq Scan on skip_fetch t1
>    ->  Materialize
>          ->  Bitmap Heap Scan on skip_fetch t2
>                Recheck Cond: (a = 1)
>                ->  Bitmap Index Scan on skip_fetch_a_idx
>                      Index Cond: (a = 1)
>
> Is it because of the shape of the index condition?  Maybe it's worth
> explaining in the comments for the tests.

There is a comment in the test that explains what it is exercising and
how. We include the explain output (the plan) to ensure it is still
using a bitmap heap scan. The test exercises the skip fetch
optimization in bitmap heap scan when not all of the inner tuples are
emitted.

Without the patch, the test fails, so it is protection against someone
adding back that assert in the future. It is not protection against
someone deleting the line
scan->rs_empty_tuples_pending = 0
That is, it doesn't verify that the empty unused tuples count is
discarded. Do you think that is necessary?

- Melanie

Attachment

v7-0001-BitmapHeapScan-Remove-incorrect-assert-and-reset-.patch

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Robert Haas

Date:

15 May, 17:24:30

Procedural comment:

It's better to get this patch committed with an imperfect test case
than to have it miss beta1.

...Robert

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Alvaro Herrera

Date:

15 May, 18:14:15

On 2024-May-14, Melanie Plageman wrote:

> On Tue, May 14, 2024 at 4:05 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> > I do wonder how do we _know_ that the test is testing what it wants to
> > test:

> We include the explain output (the plan) to ensure it is still
> using a bitmap heap scan. The test exercises the skip fetch
> optimization in bitmap heap scan when not all of the inner tuples are
> emitted.

I meant -- the query returns an empty resultset, so how do we know it's
the empty resultset that we want and not a different empty resultset
that happens to be identical?  (This is probably not a critical point
anyhow.)

> Without the patch, the test fails, so it is protection against someone
> adding back that assert in the future. It is not protection against
> someone deleting the line
> scan->rs_empty_tuples_pending = 0
> That is, it doesn't verify that the empty unused tuples count is
> discarded. Do you think that is necessary?

I don't think that's absolutely necessary.  I suspect there are
thousands of lines that you could delete that would break things, with
no test directly raising red flags.

At this point I would go with the RMT's recommendation of pushing this
now to make sure the bug is fixed for beta1, even if the test is
imperfect.  You can always polish the test afterwards if you feel like
it ...

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"No necesitamos banderas
 No reconocemos fronteras"                  (Jorge González)

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

15 June, 02:56:42

On Sun, Apr 7, 2024 at 12:17 AM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> After we decided not to pursue streaming bitmapheapscan for 17, I wanted
> to make sure we removed the prefetch code table AM violation -- since we
> weren't deleting that code. So what started out as me looking for a way
> to clean up one commit ended up becoming a much larger project. Sorry
> about that last minute code explosion! I do think there is a way to do
> it right and make it nice. Also that violation would be gone if we
> figure out how to get streaming bitmapheapscan behaving correctly.
>
> So, there's just more motivation to make streaming bitmapheapscan
> awesome for 18!

Attached v21 is the rest of the patches to make bitmap heap scan use
the read stream API.
Don't be alarmed by the 20 patches in the set. I tried to keep the
individual patches as small and easy to review as possible.

Patches 0001-0003 implement the async-friendly behavior needed both to
push down the VM lookups for prefetching and eventually to use the
read stream API.

Patches 0004-0006 add and make use of a common interface for the
shared and private (parallel and serial) bitmap iterator per Heikki's
suggestion in [1].

Patches 0008 - 0012 make new scan descriptors for bitmap table scans
and the heap AM implementation. It is not possible to remove the
layering violations mentioned for bitmap table scans in tableam.h
without passing more bitmap-specific parameters to
table_begin/end/rescan(). Because bitmap heap scans use a fairly
reduced set of the members in the HeapScanDescData, it made sense to
enable specialized scan descriptors for bitmap table scans.

0013 pushes the primary iterator setup down into heap code.

0014 and 0015 push all of the prefetch code down into heap AM code as
suggested by Heikki in [1].

0017 removes scan_bitmap_next_block() per Heikki's suggestion in [1].

After all of these patches, we've removed the layering violations
mentioned previously in tableam.h. There is no use of the visibility
map anymore in generic bitmap table scan code. Almost all
block-specific logic is gone. The table AMs own the iterator almost
completely.

The one relic of iterator ownership is that for parallel bitmap heap
scan, a single process must scan the index, construct the bitmap, and
call tbm_prepare_shared_iterate() to set up the iterator for all the
processes. I didn't see a way to push this down (because to build the
bitmap we have to scan the index and call ExecProcNode()). I wonder if
this creates an odd split of responsibilities. I could use some other
synchronization mechanism to communicate which process built the
bitmap in the generic bitmap table scan code and thus should set up
the iterator in the heap implementation, but that sounds like a pretty
bad idea. Also, I'm not sure how a non-block-based table AM would use
TBMIterators (or even TIDBitmap) anyway.

As a side note, I've started naming all new structs in the bitmap
table scan code with the `BitmapTableScan` convention. It seems like
it might be a good time to rename the executor node from
BitmapHeapScanState to BitmapTableScanState. I didn't because I
wondered if I also needed to rename the file (nodeBitmapHeapscan.c ->
nodeBitmapTablescan.c) and update all other places using the
`BitmapHeapScan` convention.

Patches 0018 and 0019 make some changes to the TIDBitmap API to
support having multiple TBMIterateResults at the same time instead of
reusing the same one when iterating. With async execution we may have
more than one TBMIterateResult at a time.

There is one MTODO in the whole patch set -- in 0019 -- related to
resetting state in the TBMIterateResult. See that patch for the full
question.

And, finally, 0020 uses the read stream API and removes all the
bespoke prefetching code from bitmap heap scan.

Assuming we all get to a happy place with the code up until 0020, the
next step is to go back and investigate the performance regression
with bitmap heap scan and the read stream API first reported by Tomas
Vondra in [2].

I'd be very happy for code review on any of the patches, answers to my
questions above, or help investigating the regression Tomas found.

- Melanie

[1] https://www.postgresql.org/message-id/5a172d1e-d69c-409a-b1fa-6521214c81c2%40iki.fi
[2] https://www.postgresql.org/message-id/5d5954ed-6f43-4f1a-8e19-ece75b2b7362@enterprisedb.com

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

18 June, 00:22:12

On Fri, Jun 14, 2024 at 7:56 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> Attached v21 is the rest of the patches to make bitmap heap scan use
> the read stream API.
---snip---
> The one relic of iterator ownership is that for parallel bitmap heap
> scan, a single process must scan the index, construct the bitmap, and
> call tbm_prepare_shared_iterate() to set up the iterator for all the
> processes. I didn't see a way to push this down (because to build the
> bitmap we have to scan the index and call ExecProcNode()). I wonder if
> this creates an odd split of responsibilities. I could use some other
> synchronization mechanism to communicate which process built the
> bitmap in the generic bitmap table scan code and thus should set up
> the iterator in the heap implementation, but that sounds like a pretty
> bad idea. Also, I'm not sure how a non-block-based table AM would use
> TBMIterators (or even TIDBitmap) anyway.

I tinkered around with this some more and actually came up with a
solution to my primary concern with the code structure. Attached is
v22. It still needs the performance regression investigation mentioned
in my previous email, but I feel more confident in the layering of the
iterator ownership I ended up with.

Because the patch numbers have changed, below is a summary of the
contents with the new patch numbers:

Patches 0001-0003 implement the async-friendly behavior needed both to
push down the VM lookups for prefetching and eventually to use the
read stream API.

Patches 0004-0006 add and make use of a common interface for the
shared and private bitmap iterators per Heikki's
suggestion in [1].

Patches 0008 - 0012 make new scan descriptors for bitmap table scans
and the heap AM implementation.

0013 and 0014 push all of the prefetch code down into heap AM code as
suggested by Heikki in [1].

0016 removes scan_bitmap_next_block() per Heikki's suggestion in [1].

Patches 0017 and 0018 make some changes to the TIDBitmap API to
support having multiple TBMIterateResults at the same time instead of
reusing the same one when iterating.

0019 uses the read stream API and removes all the bespoke prefetching
code from bitmap heap scan.

- Melanie

[1] https://www.postgresql.org/message-id/5a172d1e-d69c-409a-b1fa-6521214c81c2%40iki.fi

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Tomas Vondra

Date:

19 June, 01:02:56

Hi,

I went through v22 to remind myself of what the patches do and do some
basic review - I have some simple questions / comments for now, nothing
major. I've kept the comments in separate 'review' patches, it does not
seem worth copying here.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment

Re: BitmapHeapScan streaming read user and prelim refactoring

From

Melanie Plageman

Date:

19 June, 18:55:20

On Tue, Jun 18, 2024 at 6:02 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> I went through v22 to remind myself of what the patches do and do some
> basic review - I have some simple questions / comments for now, nothing
> major. I've kept the comments in separate 'review' patches, it does not
> seem worth copying here.

Thanks so much for the review!

I've implemented your feedback in attached v23 except for what I
mention below. I have not gone through each patch in the new set very
carefully after making the changes because I think we should resolve
the question of adding the new table scan descriptor before I do that.
A change there will require a big rebase. Then I can go through each
patch very carefully.

From your v22b-0005-review.patch:

 src/backend/executor/nodeBitmapHeapscan.c | 14 ++++++++++++++
 src/include/access/tableam.h              |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/src/backend/executor/nodeBitmapHeapscan.c
b/src/backend/executor/nodeBitmapHeapscan.c
index e8b4a754434..6d7ef9ced19 100644
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -270,6 +270,20 @@ new_page:

         BitmapAdjustPrefetchIterator(node);

+        /*
+         * XXX I'm a bit unsure if this needs to be handled using
goto. Wouldn't
+         * it be simpler / easier to understand to have two nested loops?
+         *
+         * while (true)
+         *        if (!table_scan_bitmap_next_block(...)) { break; }
+         *        while (table_scan_bitmap_next_tuple(...)) {
+         *            ... process tuples ...
+         *        }
+         *
+         * But I haven't tried implementing this.
+         */
         if (!table_scan_bitmap_next_block(scan, &node->blockno, &node->recheck,
                                           &node->lossy_pages,
&node->exact_pages))
             break;

We need to call table_scan_bimtap_next_block() the first time we call
BitmapHeapNext() on each scan but all subsequent invocations of
BitmapHeapNext() must call table_scan_bitmap_next_tuple() first each
-- because we return from BitmapHeapNext() to yield a tuple even when
there are more tuples on the page. I tried refactoring this a few
different ways and personally found the goto most clear.

From your v22b-0010-review.patch:

@@ -557,6 +559,8 @@ ExecReScanBitmapHeapScan(BitmapHeapScanState *node)
         table_rescan(node->ss.ss_currentScanDesc, NULL);

     /* release bitmaps and buffers if any */
+    /* XXX seems it should not be right after the comment, also shouldn't
+     * we still reset the prefetch_iterator field to NULL? */
     tbm_end_iterate(&node->prefetch_iterator);
     if (node->tbm)
         tbm_free(node->tbm);

prefetch_iterator is a TBMIterator which is stored in the struct (as
opposed to having a pointer to it stored in the struct).
tbm_end_iterate() sets the actual private and shared iterator pointers
to NULL.

From your v22b-0017-review.patch

diff --git a/src/include/access/relscan.h b/src/include/access/relscan.h
index 036ef29e7d5..9c711ce0eb0 100644
--- a/src/include/access/relscan.h
+++ b/src/include/access/relscan.h
@@ -52,6 +52,13 @@ typedef struct TableScanDescData
 } TableScanDescData;
 typedef struct TableScanDescData *TableScanDesc;

+/*
+ * XXX I don't understand why we should have this special node if we
+ * don't have special nodes for other scan types.

In this case, up until the final commit (using the read stream
interface), there are six fields required by bitmap heap scan that are
not needed by any other user of HeapScanDescData. There are also
several members of HeapScanDescData that are not needed by bitmap heap
scans and all of the setup in initscan() for those fields is not
required for bitmap heap scans.

Also, because the BitmapHeapScanDesc needs information like the
ParallelBitmapHeapState and prefetch_maximum (for use in prefetching),
the scan_begin() callback would have to take those as parameters. I
thought adding so much bitmap table scan-specific information to the
generic table scan callbacks was a bad idea.

Once we add the read stream API code, the number of fields required
for bitmap heap scan that are in the scan descriptor goes down to
three. So, perhaps we could justify having that many bitmap heap
scan-specific fields in the HeapScanDescData.

Though, I actually think we could start moving toward having
specialized scan descriptors if the requirements for that scan type
are appreciably different. I can't think of new code that would be
added to the HeapScanDescData that would have to be duplicated over to
specialized scan descriptors.

With the final read stream state, I can see the argument for bloating
the HeapScanDescData with three extra members and avoiding making new
scan descriptors. But, for the intermediate patches which have all of
the bitmap prefetch members pushed down into the HeapScanDescData, I
think it is really not okay. Six members only for bitmap heap scans
and two bitmap-specific members to begin_scan() seems bad.

What I thought we plan to do is commit the refactoring patches
sometime after the branch for 18 is cut and leave the final read
stream patch uncommitted so we can do performance testing on it. If
you think it is okay to have the six member bloated HeapScanDescData
in master until we push the read stream code, I am okay with removing
the BitmapTableScanDesc and BitmapHeapScanDesc.

+ * XXX Also, maybe this should do the naming convention with Data at
+ * the end (mostly for consistency).
+ */
 typedef struct BitmapTableScanDesc
 {
     Relation    rs_rd;            /* heap relation descriptor */

I really want to move away from these Data typedefs. I find them so
confusing as a developer, but it's hard to justify ripping out the
existing ones because of code churn. If we add new scan descriptors, I
had really hoped to start using a different pattern.

From your v22b-0025-review.patch

diff --git a/src/backend/access/table/tableamapi.c
b/src/backend/access/table/tableamapi.c
index a47527d490a..379b7df619e 100644
--- a/src/backend/access/table/tableamapi.c
+++ b/src/backend/access/table/tableamapi.c
@@ -91,6 +91,9 @@ GetTableAmRoutine(Oid amhandler)

     Assert(routine->relation_estimate_size != NULL);

+    /* XXX shouldn't this check that _block is not set without _tuple?
+     * Also, the commit message says _block is "local helper" but then
+     * why would it be part of TableAmRoutine? */
     Assert(routine->scan_sample_next_block != NULL);
     Assert(routine->scan_sample_next_tuple != NULL);

scan_bitmap_next_block() is removed as a table AM callback here, so we
don't check if it is set. We do still check if scan_bitmap_next_tuple
is set if amgetbitmap is not NULL. heapam_scan_bitmap_next_block() is
now a local helper for heapam_scan_bitmap_next_tuple(). Perhaps I
should change the name to something like heap_scan_bitmap_next_block()
to make it clear it is not an implementation of a table AM callback?

- Melanie

On Thu, Dec 19, 2024 at 10:23 AM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Thu, Dec 19, 2024 at 10:12 AM Richard Guo <guofenglinux@gmail.com> wrote:
> >
> > On Thu, Dec 19, 2024 at 6:15 PM Richard Guo <guofenglinux@gmail.com> wrote:
> > > I think we need to check whether rs_tbmiterator is NULL before calling
> > > tbm_end_iterate on it, like below.
> > >
> > > --- a/src/backend/executor/nodeBitmapHeapscan.c
> > > +++ b/src/backend/executor/nodeBitmapHeapscan.c
> > > @@ -572,9 +572,11 @@ ExecReScanBitmapHeapScan(BitmapHeapScanState *node)
> > >     if (scan)
> > >     {
> > >         /*
> > > -        * End iteration on iterators saved in scan descriptor.
> > > +        * End iteration on iterators saved in scan descriptor, if they
> > > +        * haven't already been cleaned up.
> > >          */
> > > -       tbm_end_iterate(&scan->st.rs_tbmiterator);
> > > +       if (!tbm_exhausted(&scan->st.rs_tbmiterator))
> > > +           tbm_end_iterate(&scan->st.rs_tbmiterator);
> > >
> > >         /* rescan to release any page pin */
> > >         table_rescan(node->ss.ss_currentScanDesc, NULL);
> >
> > This change may also be needed in ExecEndBitmapHeapScan().
>
> Thanks, Richard! I'm working on the fix and adding the test case you found.

Okay, pushed. Thanks so much, Richard. I also noticed another
oversight and fixed it.

- Melanie