Re: [HACKERS] Patch: Write Amplification Reduction Method (WARM) - Mailing list pgsql-hackers

On Wed, Mar 22, 2017 at 4:06 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Tue, Mar 21, 2017 at 6:47 PM, Pavan Deolasee
> <pavan.deolasee@gmail.com> wrote:
>>
>>>
>>
>> Please find attached rebased patches.
>>
>
> Few comments on 0005_warm_updates_v18.patch:
>

Few more comments on 0005_warm_updates_v18.patch:
1.
@@ -234,6 +241,25 @@ index_beginscan(Relation heapRelation,
  scan->heapRelation = heapRelation;
  scan->xs_snapshot = snapshot;

+ /*
+ * If the index supports recheck,
make sure that index tuple is saved
+ * during index scans. Also build and cache IndexInfo which is used by
+ * amrecheck routine.
+ *
+ * XXX Ideally, we should look at
all indexes on the table and check if
+ * WARM is at all supported on the base table. If WARM is not supported
+ * then we don't need to do any recheck.
RelationGetIndexAttrBitmap() does
+ * do that and sets rd_supportswarm after looking at all indexes. But we
+ * don't know if the function was called earlier in the
session when we're
+ * here. We can't call it now because there exists a risk of causing
+ * deadlock.
+ */
+ if (indexRelation->rd_amroutine->amrecheck)
+ {
+scan->xs_want_itup = true;
+ scan->indexInfo = BuildIndexInfo(indexRelation);
+ }
+

Don't we need to do this rechecking during parallel scans?  Also what
about bitmap heap scans?

2.
+++ b/src/backend/access/nbtree/nbtinsert.c
-
 typedef struct

Above change is not require.

3.
+_bt_clear_items(Page page, OffsetNumber *clearitemnos, uint16 nclearitems)
+void _hash_clear_items(Page page, OffsetNumber *clearitemnos,
+   uint16 nclearitems)

Both the above functions look exactly same, isn't it better to have a
single function like page_clear_items?  If you want separation for
different index types, then we can have one common function which can
be called from different index types.

4.
- if (callback(htup, callback_state))
+ flags = ItemPointerGetFlags(&itup->t_tid);
+ is_warm = ((flags & BTREE_INDEX_WARM_POINTER) != 0);
+
+ if (is_warm)
+ stats->num_warm_pointers++;
+ else
+ stats->num_clear_pointers++;
+
+ result = callback(htup, is_warm, callback_state);
+ if (result == IBDCR_DELETE)
+ {
+ if (is_warm)
+ stats->warm_pointers_removed++;
+ else
+ stats->clear_pointers_removed++;

The patch looks to be inconsistent in collecting stats for btree and
hash.  I don't see above stats are getting updated in hash index code.

5.
+btrecheck(Relation indexRel, IndexInfo *indexInfo, IndexTuple indexTuple,
+ Relation heapRel, HeapTuple heapTuple)
{
..
+ if (!datumIsEqual(values[i - 1], indxvalue, att->attbyval,
+ att->attlen))
..
}

Will this work if the index is using non-default collation?

6.
+++ b/src/backend/access/nbtree/nbtxlog.c
@@ -390,83 +390,9 @@ btree_xlog_vacuum(XLogReaderState *record)
-#ifdef UNUSED
  xl_btree_vacuum *xlrec = (xl_btree_vacuum *) XLogRecGetData(record);

  /*
- * This section of code is thought to be no longer needed, after analysis
- * of the calling paths. It is retained to allow the code to be reinstated
- * if a flaw is revealed in that thinking.
- *
..

Why does this patch need to remove the above code under #ifdef UNUSED