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Commit23a0cf2

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Use TransactionXmin instead of RecentGlobalXmin in heap_abort_speculative().
There's a very low risk that RecentGlobalXmin could be far enough inthe past to be older than relfrozenxid, or even wrappedaround. Luckily the consequences of that having happened wouldn't betoo bad - the page wouldn't be pruned for a while.Avoid that risk by using TransactionXmin instead. As that's announcedvia MyPgXact->xmin, it is protected against wrapping around (see codecomments for details around relfrozenxid).Author: Andres FreundDiscussion:https://postgr.es/m/20200328213023.s4eyijhdosuc4vcj@alap3.anarazel.deBackpatch: 9.5-
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‎src/backend/access/heap/heapam.c

Lines changed: 15 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -5572,6 +5572,7 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
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Pagepage;
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BlockNumberblock;
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Bufferbuffer;
5575+
TransactionIdprune_xid;
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55765577
Assert(ItemPointerIsValid(tid));
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@@ -5614,13 +5615,21 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
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START_CRIT_SECTION();
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/*
5617-
* The tuple will become DEAD immediately. Flag that this page
5618-
* immediately is a candidate for pruning by setting xmin to
5619-
* RecentGlobalXmin. That's not pretty, but it doesn't seem worth
5620-
* inventing a nicer API for this.
5618+
* The tuple will become DEAD immediately. Flag that this page is a
5619+
* candidate for pruning by setting xmin to TransactionXmin. While not
5620+
* immediately prunable, it is the oldest xid we can cheaply determine
5621+
* that's safe against wraparound / being older than the table's
5622+
* relfrozenxid. To defend against the unlikely case of a new relation
5623+
* having a newer relfrozenxid than our TransactionXmin, use relfrozenxid
5624+
* if so (vacuum can't subsequently move relfrozenxid to beyond
5625+
* TransactionXmin, so there's no race here).
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*/
5622-
Assert(TransactionIdIsValid(RecentGlobalXmin));
5623-
PageSetPrunable(page,RecentGlobalXmin);
5627+
Assert(TransactionIdIsValid(TransactionXmin));
5628+
if (TransactionIdPrecedes(TransactionXmin,relation->rd_rel->relfrozenxid))
5629+
prune_xid=relation->rd_rel->relfrozenxid;
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else
5631+
prune_xid=TransactionXmin;
5632+
PageSetPrunable(page,prune_xid);
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/* store transaction information of xact deleting the tuple */
56265635
tp.t_data->t_infomask &= ~(HEAP_XMAX_BITS |HEAP_MOVED);

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