From e76a68f196d96f269d5719b46d8ba1c9b8950870 Mon Sep 17 00:00:00 2001 From: Melanie Plageman Date: Wed, 28 Jan 2026 12:51:26 -0500 Subject: [PATCH v34 01/14] Clarify some heap tuple freezing related comments Some of the comments about calculating the snapshot conflict horizon for a record freezing tuples needed further elaboration. --- src/backend/access/heap/pruneheap.c | 30 +++++++++++++++++++---------- 1 file changed, 20 insertions(+), 10 deletions(-) diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c index 632c2427952..b9d2b48104c 100644 --- a/src/backend/access/heap/pruneheap.c +++ b/src/backend/access/heap/pruneheap.c @@ -736,17 +736,26 @@ heap_page_will_freeze(Relation relation, Buffer buffer, heap_pre_freeze_checks(buffer, prstate->frozen, prstate->nfrozen); /* - * Calculate what the snapshot conflict horizon should be for a record - * freezing tuples. We can use the visibility_cutoff_xid as our cutoff - * for conflicts when the whole page is eligible to become all-frozen - * in the VM once we're done with it. Otherwise, we generate a + * Determine the snapshot conflict horizon for freezing tuples on the + * page. + * + * We don't track the newest xmin that will become frozen, so we must + * use a coarser (more conservative) cutoff as the conflict horizon. + * + * We can use the visibility_cutoff_xid as our cutoff for conflicts + * when the whole page is eligible to become all-frozen in the VM once + * we're done with it. Otherwise, we generate an even more * conservative cutoff by stepping back from OldestXmin. + * + * Ignoring dead items when all other tuples will be frozen allows us + * to pick an older horizon (visibility_cutoff_xid will be older than + * OldestXmin). We will later adjust this horizon to account for dead + * items, moving it forward if the newest removed xid is newer. */ if (prstate->all_frozen) prstate->frz_conflict_horizon = prstate->visibility_cutoff_xid; else { - /* Avoids false conflicts when hot_standby_feedback in use */ prstate->frz_conflict_horizon = prstate->cutoffs->OldestXmin; TransactionIdRetreat(prstate->frz_conflict_horizon); } @@ -877,11 +886,12 @@ heap_page_prune_and_freeze(PruneFreezeParams *params, /* * While scanning the line pointers, we did not clear * all_visible/all_frozen when encountering LP_DEAD items because we - * wanted the decision whether or not to freeze the page to be unaffected - * by the short-term presence of LP_DEAD items. These LP_DEAD items are - * effectively assumed to be LP_UNUSED items in the making. It doesn't - * matter which vacuum heap pass (initial pass or final pass) ends up - * setting the page all-frozen, as long as the ongoing VACUUM does it. + * wanted the decision whether or not to opportunistically freeze the page + * to be unaffected by the short-term presence of LP_DEAD items. These + * LP_DEAD items are effectively assumed to be LP_UNUSED items in the + * making. It doesn't matter which vacuum heap pass (initial pass or final + * pass) ends up setting the page all-frozen, as long as the ongoing + * VACUUM does it. * * Now that we finished determining whether or not to freeze the page, * update all_visible and all_frozen so that they reflect the true state -- 2.43.0