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From: Melanie Plageman <[email protected]>
Subject: [PATCH v18 04/10] Use prefetch block recheck value to determine skip_fetch
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 should skip prefetching
the proposed prefetch block. As explained in the comment, this assumed
the index AM will report the same recheck value for the future page as
it did for the current page - but there's no guarantee.

This only affects prefetching - we may prefetch blocks unecessarily and
not prefetch blocks that will be needed. But we don't need to rely on
that assumption - we know the recheck flag for the block we're
considering prefetching.

Author: Melanie Plageman
Reviewed-by: Tomas Vondra, Andres Freund, Tom Lane
Discussion: https://postgr.es/m/1939305.1712415547%40sss.pgh.pa.us
---
 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 5f768701a5e..6f843908032 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_TUPLES) &&
-							  (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_TUPLES) &&
-							  (node->tbmres ? !node->tbmres->recheck : false) &&
+							  !tbmpre->recheck &&
 							  VM_ALL_VISIBLE(node->ss.ss_currentRelation,
 											 tbmpre->blockno,
 											 &node->pvmbuffer));
-- 
2.40.1


--zmcd37za4qx3kv73
Content-Type: text/x-diff; charset=us-ascii
Content-Disposition: attachment;
	filename="v18-0005-Change-BitmapAdjustPrefetchIterator-to-accept-Bl.patch"

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