Re: Incremental sorts and EXEC_FLAG_REWIND - Mailing list pgsql-hackers

On 4/24/20 6:57 PM, Tomas Vondra wrote:
> On Fri, Apr 24, 2020 at 04:35:02PM -0400, James Coleman wrote:
>> On Sun, Apr 19, 2020 at 12:14 PM James Coleman <jtc331@gmail.com> wrote:
>>>
>>> On Wed, Apr 15, 2020 at 2:04 PM James Coleman <jtc331@gmail.com> wrote:
>>> >
>>> > On Wed, Apr 15, 2020 at 11:02 AM James Coleman <jtc331@gmail.com>
>>> wrote:
>>> > >
>>> > > On Tue, Apr 14, 2020 at 2:53 AM Michael Paquier
>>> <michael@paquier.xyz> wrote:
>>> > > >
>>> > > > Hi,
>>> > > >
>>> > > > When initializing an incremental sort node, we have the
>>> following as
>>> > > > of ExecInitIncrementalSort():
>>> > > >     /*
>>> > > >      * Incremental sort can't be used with either
>>> EXEC_FLAG_REWIND,
>>> > > >      * EXEC_FLAG_BACKWARD or EXEC_FLAG_MARK, because we only
>>> one of many sort
>>> > > >      * batches in the current sort state.
>>> > > >      */
>>> > > >      Assert((eflags & (EXEC_FLAG_BACKWARD |
>>> > > >                        EXEC_FLAG_MARK)) == 0);
>>> > > > While I don't quite follow why EXEC_FLAG_REWIND should be
>>> allowed here
>>> > > > to begin with (because incremental sorts don't support rescans
>>> without
>>> > > > parameter changes, right?), the comment and the assertion are
>>> telling
>>> > > > a different story.
>>> > >
>>> > > I remember changing this assertion in response to an issue I'd found
>>> > > which led to rewriting the rescan implementation, but I must have
>>> > > missed updating the comment.
>>> >
>>> > All right, here are the most relevant messages:
>>> >
>>> > [1]: Here I'd said:
>>> > ----------
>>> > While working on finding a test case to show rescan isn't implemented
>>> > properly yet, I came across a bug. At the top of
>>> > ExecInitIncrementalSort, we assert that eflags does not contain
>>> > EXEC_FLAG_REWIND. But the following query (with merge and hash joins
>>> > disabled) breaks that assertion:
>>> >
>>> > select * from t join (select * from t order by a, b) s on s.a = t.a
>>> > where t.a in (1,2);
>>> >
>>> > The comments about this flag in src/include/executor/executor.h say:
>>> >
>>> > * REWIND indicates that the plan node should try to efficiently
>>> support
>>> > * rescans without parameter changes. (Nodes must support ExecReScan
>>> calls
>>> > * in any case, but if this flag was not given, they are at liberty
>>> to do it
>>> > * through complete recalculation. Note that a parameter change
>>> forces a
>>> > * full recalculation in any case.)
>>> >
>>> > Now we know that except in rare cases (as just discussed recently up
>>> > thread) we can't implement rescan efficiently.
>>> >
>>> > So is this a planner bug (i.e., should we try not to generate
>>> > incremental sort plans that require efficient rewind)? Or can we just
>>> > remove that part of the assertion and know that we'll implement the
>>> > rescan, albeit inefficiently? We already explicitly declare that we
>>> > don't support backwards scanning, but I don't see a way to declare the
>>> > same for rewind.
>>> > ----------
>>> >
>>> > So it seems to me that we can't disallow REWIND, and we have to
>>> > support rescan, but, we could try to mitigate the effects (without a
>>> > param change) with a materialize node, as noted below.
>>> >
>>> > [2]: Here, in response to my questioning above if this was a planner
>>> > bug, I'd said:
>>> > ----------
>>> > Other nodes seem to get a materialization node placed above them to
>>> > support this case "better". Is that something we should be doing?
>>> > ----------
>>> >
>>> > I never got any reply on this point; if we _did_ introduce a
>>> > materialize node here, then it would mean we could start disallowing
>>> > REWIND again. See the email for full details of a specific plan that I
>>> > encountered that reproduced this.
>>> >
>>> > Thoughts?
>>> >
>>> > > In the meantime, your question is primarily about making sure the
>>> > > code/comments/etc. are consistent and not a behavioral problem or
>>> > > failure you've seen in testing?
>>> >
>>> > Still want to confirm this is the case.
>>> >
>>> > James
>>> >
>>> > [1]:
>>> https://www.postgresql.org/message-id/CAAaqYe9%2Bap2SbU_E2WaC4F9ZMF4oa%3DpJZ1NBwaKDMP6GFUA77g%40mail.gmail.com
>>>
>>> > [2]:
>>> https://www.postgresql.org/message-id/CAAaqYe-sOp2o%3DL7nvGZDJ6GsL9%3Db_ztrGE1rhyi%2BF82p3my2bQ%40mail.gmail.com
>>>
>>>
>>> Looking at this more, I think this is definitely suspect. The current
>>> code shields lower nodes from EXEC_FLAG_BACKWARD and EXEC_FLAG_MARK --
>>> the former is definitely fine because we declare that we don't support
>>> backwards scans. The latter seems like the same reasoning would apply,
>>> but unfortunately we didn't add it to ExecSupportsMarkRestore, so I've
>>> attached a patch to do that.
>>>
>>> The EXEC_FLAG_REWIND situation though I'm still not clear on -- given
>>> the comments/docs seem to suggest it's a hint for efficiency rather
>>> than something that has to work or be declared as not implemented, so
>>> it seems like one of the following should be the outcome:
>>>
>>> 1. "Disallow" it by only generating materialize nodes above the
>>> incremental sort node if REWIND will be required. I'm not sure if this
>>> would mean that incremental sort just wouldn't be useful in that case?
>>> 2. Keep the existing implementation where we basically ignore REWIND
>>> and use our more inefficient implementation. In this case, I believe
>>> we need to stop shielding child nodes from REWIND, though, since we we
>>> aren't actually storing the full result set and will instead be
>>> re-executing the child nodes.
>>>
>>> I've attached a patch to take course (2), since it's the easiest to
>>> implement. But I'd still like feedback on what we should do here,
>>> because I don't feel like I actually know what the semantics expected
>>> of the executor/planner are on this point. If we do go with this
>>> approach, someone should verify my comments additions about
>>> materialize nodes is correct.
>>
>> I also happened to noticed that in rescan we are always setting
>> node->bounded = false. I was under the impression that
>> ExecSetTupleBound would be called *after* ExecReScanIncrementalSort,
>> but looking at both ExecSetTupleBound and ExecReScanSort, but it seems
>> that the inverse is true. Therefore if we set this to false each time,
>> then we lose any possibility of using the bounded optimization for all
>> rescans.
>>
>> I've added a tiny patch (minus one line) to the earlier patch series
>> to fix that.
>>
>
> Thanks. I'll take a look at the issue and fixes.

With Beta 1 just around the corner[1], I wanted to check in to see if
this was closer to being committed so we could close off the open
item[2] prior the beta release.

Thanks!

Jonathan

[1]
https://www.postgresql.org/message-id/b782a4ec-5e8e-21a7-f628-624be683e6d6@postgresql.org
[2] https://wiki.postgresql.org/wiki/PostgreSQL_13_Open_Items