From 796e139454a8863c12235d248d76f313da6ef002 Mon Sep 17 00:00:00 2001 From: Melanie Plageman Date: Mon, 12 Feb 2024 19:03:24 -0500 Subject: [PATCH v16 04/18] BitmapPrefetch use prefetch block recheck for skip fetch 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