From 8b527ff196b8752809b92f7073bb7624d2329170 Mon Sep 17 00:00:00 2001 From: Melanie Plageman Date: Thu, 29 May 2025 10:39:35 -0400 Subject: [PATCH] Calculate freeze conflict horizon with final all_frozen value In phase I of heap vacuuming, while determining whether or not to freeze tuples on a page, lazy_scan_prune() ignores LP_DEAD items which it assumes will be set LP_UNUSED and removed by the phase III of vacuum. That means the local value of prunestate->all_frozen it uses may be true even when dead items are present. In this case, however, before returning control to lazy_scan_heap() lazy_scan_prune() must unset prunsteate->all_visible to avoid lazy_scan_haep() incorrectly updating the visibility map for that heap page. The problem was that lazy_scan_prune() calculated the snapshot conflict horizon for the freeze record using the contingent value of prunestate->all_frozen. That could result in an overly aggressive snapshot conflict horizon in the case that some tuples were frozen but the whole page was not able to be set all-frozen. This would not affect correctness -- only potentially cancel queries unnecessarily on the standby. Unset this sooner to correctly set the snapshot conflict horizon in the freeze record. This is the minimal change to achieve this affect in backbranches, however, the value of all_frozen is not corrected before returning to lazy_scan_prune(). That doesn't affect correctness since it is not used without all_visible. It seemed wiser to do the smallest possible fix. --- src/backend/access/heap/vacuumlazy.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c index 9fa88960ada..8c3605eff1f 100644 --- a/src/backend/access/heap/vacuumlazy.c +++ b/src/backend/access/heap/vacuumlazy.c @@ -1833,7 +1833,7 @@ retry: * once we're done with it. Otherwise we generate a conservative * cutoff by stepping back from OldestXmin. */ - if (prunestate->all_visible && prunestate->all_frozen) + if (prunestate->all_visible && prunestate->all_frozen && lpdead_items == 0) { /* Using same cutoff when setting VM is now unnecessary */ snapshotConflictHorizon = prunestate->visibility_cutoff_xid; -- 2.34.1