From f7f55d02c2a1e854f21cde4124453c2b247b701f 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 fe3e83b4894..cfc340834be 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -6152,6 +6152,7 @@ heap_abort_speculative(Relation relation, HeapTuple tuple) Page page; BlockNumber block; Buffer buffer; + TransactionId prune_xid; Assert(ItemPointerIsValid(tid)); @@ -6194,13 +6195,21 @@ heap_abort_speculative(Relation relation, HeapTuple tuple) 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);