Subject: [PATCH v16 04/18] BitmapPrefetch use prefetch block recheck for skip fetch

From: Melanie Plageman <[email-masked]>
Date: Mon, 12 Feb 2024 19:03:24 -0500

As of 7c70996ebf0949b142a9, BitmapPrefetch() used the recheck flag for
the current block to determine whether or not it could skip prefetching
the proposed prefetch block. This doesn't seem right. We should use the
prefetch block's recheck flag to determine whether or not to prefetch
it. The current block's recheck flag has no relationship to the prefetch
block's recheck flag. See this [1] thread on hackers reporting the
issue.

[1] https://www.postgresql.org/message-id/CAAKRu_bxrXeZ2rCnY8LyeC2Ls88KpjWrQ%2BopUrXDRXdcfwFZGA%40mail.gmail.com

Author: Melanie Plageman
Reviewed-by: Andres Freund, Heikki Linnakangas, Tomas Vondra
Discussion: https://postgr.es/m/CAAKRu_ZwCwWFeL_H3ia26bP2e7HiKLWt0ZmGXPVwPO6uXq0vaA%40mail.gmail.com
---
 src/backend/executor/nodeBitmapHeapscan.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
index a8bc5dec53..2e2cec8b3b 100644
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -475,14 +475,9 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
 				 * skip this prefetch call, but continue to run the prefetch
 				 * logic normally.  (Would it be better not to increment
 				 * prefetch_pages?)
-				 *
-				 * This depends on the assumption that the index AM will
-				 * report the same recheck flag for this future heap page as
-				 * it did for the current heap page; which is not a certainty
-				 * but is true in many cases.
 				 */
 				skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLE) &&
-							  (node->tbmres ? !node->tbmres->recheck : false) &&
+							  !tbmpre->recheck &&
 							  VM_ALL_VISIBLE(node->ss.ss_currentRelation,
 											 tbmpre->blockno,
 											 &node->pvmbuffer));
@@ -533,7 +528,7 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
 
 				/* As above, skip prefetch if we expect not to need page */
 				skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLE) &&
-							  (node->tbmres ? !node->tbmres->recheck : false) &&
+							  !tbmpre->recheck &&
 							  VM_ALL_VISIBLE(node->ss.ss_currentRelation,
 											 tbmpre->blockno,
 											 &node->pvmbuffer));
-- 
2.40.1

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	filename="v16-0005-Update-BitmapAdjustPrefetchIterator-parameter-ty.patch"