From f946069e6827e729857b9f2db06bf27a1c0563ee Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Sun, 5 Apr 2020 17:47:30 -0700 Subject: [PATCH] Use TransactionXmin instead of RecentGlobalXmin in heap_abort_speculative(). There's a very low risk that RecentGlobalXmin could be far enough in the past to be older than relfrozenxid, or even wrapped around. Luckily the consequences of that having happened wouldn't be too bad - the page wouldn't be pruned for a while. Avoid that risk by using TransactionXmin instead. As that's announced via MyPgXact->xmin, it is protected against wrapping around (see code comments for details around relfrozenxid). Author: Andres Freund Discussion: https://postgr.es/m/20200328213023.s4eyijhdosuc4vcj@alap3.anarazel.de Backpatch: 9.5- --- src/backend/access/heap/heapam.c | 21 +++++++++++++++------ 1 file changed, 15 insertions(+), 6 deletions(-) diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index a25d539ec47..c4a5aa616a3 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -5581,6 +5581,7 @@ heap_abort_speculative(Relation relation, ItemPointer tid) Page page; BlockNumber block; Buffer buffer; + TransactionId prune_xid; Assert(ItemPointerIsValid(tid)); @@ -5623,13 +5624,21 @@ heap_abort_speculative(Relation relation, ItemPointer tid) START_CRIT_SECTION(); /* - * The tuple will become DEAD immediately. Flag that this page - * immediately is a candidate for pruning by setting xmin to - * RecentGlobalXmin. That's not pretty, but it doesn't seem worth - * inventing a nicer API for this. + * The tuple will become DEAD immediately. Flag that this page is a + * candidate for pruning by setting xmin to TransactionXmin. While not + * immediately prunable, it is the oldest xid we can cheaply determine + * that's safe against wraparound / being older than the table's + * relfrozenxid. To defend against the unlikely case of a new relation + * having a newer relfrozenxid than our TransactionXmin, use relfrozenxid + * if so (vacuum can't subsequently move relfrozenxid to beyond + * TransactionXmin, so there's no race here). */ - Assert(TransactionIdIsValid(RecentGlobalXmin)); - PageSetPrunable(page, RecentGlobalXmin); + Assert(TransactionIdIsValid(TransactionXmin)); + if (TransactionIdPrecedes(TransactionXmin, relation->rd_rel->relfrozenxid)) + prune_xid = relation->rd_rel->relfrozenxid; + else + prune_xid = TransactionXmin; + PageSetPrunable(page, prune_xid); /* store transaction information of xact deleting the tuple */ tp.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);