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, Alvaro Herrera
Reported-by: Melanie Plageman
Reproduced-by: Tomas Vondra, Richard Guo
Discussion: https://postgr.es/m/CAMbWs48orzZVXa7-vP9Nt7vQWLTE04Qy4PePaLQYsVNQgo6qRg%40mail.gmail.com

1 files changed, 6 insertions, 3 deletions

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 4be0dee4de0..82bb9cb33b6 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.
 	 */