BitmapHeapScan: Remove incorrect assert and reset field

04e72ed617 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.

04e72ed617 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, Alvaro Herrera
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

@@ -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.
 	 */

src/test/regress/expected/join.out

@@ -7924,3 +7924,39 @@ 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_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
+   ->  Materialize
+         ->  Bitmap Heap Scan on skip_fetch t2
+               Recheck Cond: (a = 1)
+               ->  Bitmap Index Scan on skip_fetch_a_idx
+                     Index Cond: (a = 1)
+(7 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_seqscan;

src/test/regress/sql/join.sql

@@ -2904,3 +2904,27 @@ 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_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_seqscan;