Remove HeapBitmapScan's skip_fetch optimization

The optimization does not take the removal of TIDs by a concurrent vacuum into
account. The concurrent vacuum can remove dead TIDs and make pages ALL_VISIBLE
while those dead TIDs are referenced in the bitmap. This can lead to a
skip_fetch scan returning too many tuples.

It likely would be possible to implement this optimization safely, but we
don't have the necessary infrastructure in place. Nor is it clear that it's
worth building that infrastructure, given how limited the skip_fetch
optimization is.

In the backbranches we just disable the optimization by always passing
need_tuples=true to table_beginscan_bm(). We can't perform API/ABI changes in
the backbranches and we want to make the change as minimal as possible.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reported-By: Konstantin Knizhnik <knizhnik@garret.ru>
Discussion: https://postgr.es/m/CAEze2Wg3gXXZTr6_rwC+s4-o2ZVFB5F985uUSgJTsECx6AmGcQ@mail.gmail.com
Backpatch-through: 13

1 files changed, 1 insertions, 14 deletions

diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
index 3e33360c0fc..bf24f3d7fe0 100644
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -105,24 +105,11 @@ BitmapTableScanSetup(BitmapHeapScanState *node)
 	 */
 	if (!node->ss.ss_currentScanDesc)
 	{
-		bool		need_tuples = false;
-
-		/*
-		 * We can potentially skip fetching heap pages if we do not need any
-		 * columns of the table, either for checking non-indexable quals or
-		 * for returning data.  This test is a bit simplistic, as it checks
-		 * the stronger condition that there's no qual or return tlist at all.
-		 * But in most cases it's probably not worth working harder than that.
-		 */
-		need_tuples = (node->ss.ps.plan->qual != NIL ||
-					   node->ss.ps.plan->targetlist != NIL);
-
 		node->ss.ss_currentScanDesc =
 			table_beginscan_bm(node->ss.ss_currentRelation,
 							   node->ss.ps.state->es_snapshot,
 							   0,
-							   NULL,
-							   need_tuples);
+							   NULL);
 	}
 
 	node->ss.ss_currentScanDesc->st.rs_tbmiterator = tbmiterator;