On Tue, May 14, 2024 at 2:18 AM Michael Paquier <mich...@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
From d44679397e3aaa043afad2108ec904f68b7052b3 Mon Sep 17 00:00:00 2001 From: Melanie Plageman <melanieplage...@gmail.com> Date: Tue, 14 May 2024 13:36:42 -0400 Subject: [PATCH v5] BitmapHeapScan: Remove incorrect assert and reset field 04e72ed617be pushed the skip fetch optimization (allowing bitmap heap scans to operate like index-only scans if none of the underlying data is needed) into heap AM implementations of bitmap table scan callbacks. 04e72ed617be added an assert that all tuples in blocks eligible for the optimization had been NULL-filled and emitted by the end of the scan. This assert is incorrect when not all tuples need be scanned to execute the query; for example: a join in which not all inner tuples need to be scanned before skipping to the next outer tuple. 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. Author: Melanie Plageman Reviewed-by: Tomas Vondra, Michael Paquier Reported-by: Melanie Plageman Reproduced-by: Tomas Vondra, Richard Guo Discussion: https://postgr.es/m/CAMbWs48orzZVXa7-vP9Nt7vQWLTE04Qy4PePaLQYsVNQgo6qRg%40mail.gmail.com --- src/backend/access/heap/heapam.c | 9 ++++--- src/test/regress/expected/join.out | 39 ++++++++++++++++++++++++++++++ src/test/regress/sql/join.sql | 28 +++++++++++++++++++++ 3 files changed, 73 insertions(+), 3 deletions(-) diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index 4be0dee4de..82bb9cb33b 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -1184,7 +1184,12 @@ heap_rescan(TableScanDesc sscan, ScanKey key, bool set_params, scan->rs_vmbuffer = InvalidBuffer; } - Assert(scan->rs_empty_tuples_pending == 0); + /* + * Reset rs_empty_tuples_pending, a field only used by bitmap heap scan, + * to avoid incorrectly emitting NULL-filled tuples from a previous scan + * on rescan. + */ + scan->rs_empty_tuples_pending = 0; /* * The read stream is reset on rescan. This must be done before @@ -1216,8 +1221,6 @@ heap_endscan(TableScanDesc sscan) if (BufferIsValid(scan->rs_vmbuffer)) ReleaseBuffer(scan->rs_vmbuffer); - Assert(scan->rs_empty_tuples_pending == 0); - /* * Must free the read stream before freeing the BufferAccessStrategy. */ diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out index 0246d56aea..466be7b439 100644 --- a/src/test/regress/expected/join.out +++ b/src/test/regress/expected/join.out @@ -7924,3 +7924,42 @@ where exists (select 1 from j3 (13 rows) drop table j3; +-- Exercise the "skip fetch" Bitmap Heap Scan optimization when candidate +-- tuples are discarded. This may occur when: +-- 1. A join doesn't require all inner tuples to be scanned for each outer +-- tuple, and +-- 2. The inner side is scanned using a bitmap heap scan, and +-- 3. The bitmap heap scan is eligible for the "skip fetch" optimization. +-- This optimization is usable when no data from the underlying table is +-- needed. Use a temp table so it is only visible to this backend and +-- vacuum may reliably mark all blocks in the table all visible in the +-- visibility map. +CREATE TEMP TABLE skip_fetch (a INT, b INT) WITH (fillfactor=10); +INSERT INTO skip_fetch SELECT i % 3, i FROM generate_series(0,30) i; +CREATE INDEX ON skip_fetch(a); +VACUUM (ANALYZE) skip_fetch; +SET enable_indexonlyscan = off; +SET enable_indexscan = off; +SET enable_material = off; +SET enable_seqscan = off; +EXPLAIN (COSTS OFF) +SELECT t1.a FROM skip_fetch t1 LEFT JOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS NULL; + QUERY PLAN +--------------------------------------------------- + Nested Loop Anti Join + -> Seq Scan on skip_fetch t1 + -> Bitmap Heap Scan on skip_fetch t2 + Recheck Cond: (a = 1) + -> Bitmap Index Scan on skip_fetch_a_idx + Index Cond: (a = 1) +(6 rows) + +SELECT t1.a FROM skip_fetch t1 LEFT JOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS NULL; + a +--- +(0 rows) + +RESET enable_indexonlyscan; +RESET enable_indexscan; +RESET enable_material; +RESET enable_seqscan; diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql index 923e7c5549..1bd4ed8d29 100644 --- a/src/test/regress/sql/join.sql +++ b/src/test/regress/sql/join.sql @@ -2904,3 +2904,31 @@ where exists (select 1 from j3 and t1.unique1 < 1; drop table j3; + +-- Exercise the "skip fetch" Bitmap Heap Scan optimization when candidate +-- tuples are discarded. This may occur when: +-- 1. A join doesn't require all inner tuples to be scanned for each outer +-- tuple, and +-- 2. The inner side is scanned using a bitmap heap scan, and +-- 3. The bitmap heap scan is eligible for the "skip fetch" optimization. +-- This optimization is usable when no data from the underlying table is +-- needed. Use a temp table so it is only visible to this backend and +-- vacuum may reliably mark all blocks in the table all visible in the +-- visibility map. +CREATE TEMP TABLE skip_fetch (a INT, b INT) WITH (fillfactor=10); +INSERT INTO skip_fetch SELECT i % 3, i FROM generate_series(0,30) i; +CREATE INDEX ON skip_fetch(a); +VACUUM (ANALYZE) skip_fetch; + +SET enable_indexonlyscan = off; +SET enable_indexscan = off; +SET enable_material = off; +SET enable_seqscan = off; +EXPLAIN (COSTS OFF) +SELECT t1.a FROM skip_fetch t1 LEFT JOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS NULL; +SELECT t1.a FROM skip_fetch t1 LEFT JOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS NULL; + +RESET enable_indexonlyscan; +RESET enable_indexscan; +RESET enable_material; +RESET enable_seqscan; -- 2.34.1
