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Commit37b5c5f

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Avoid killing btree items that are already dead
_bt_killitems marks btree items dead when a scan leaves the page wherethey live, but it does so with only share lock (to improve concurrency).This was historicall okay, since killing a dead item has noconsequences. However, with the advent of data checksums andwal_log_hints, this action incurs a WAL full-page-image record of thepage. Multiple concurrent processes would write the same page severaltimes, leading to WAL bloat. The probability of this happening can bereduced by only killing items if they're not already dead, so change thecode to do that.The problem could eliminated completely by having _bt_killitems upgradeto exclusive lock upon seeing a killable item, but that would reduceconcurrency so it's considered a cure worse than the disease.Backpatch all the way back to 9.5, since wal_log_hints was introduced in9.4.Author: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>Discussion:https://postgr.es/m/CA+fd4k6PeRj2CkzapWNrERkja5G0-6D-YQiKfbukJV+qZGFZ_Q@mail.gmail.com
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‎src/backend/access/nbtree/nbtutils.c

Lines changed: 13 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1815,9 +1815,19 @@ _bt_killitems(IndexScanDesc scan)
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if (ItemPointerEquals(&ituple->t_tid,&kitem->heapTid))
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{
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/* found the item */
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ItemIdMarkDead(iid);
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killedsomething= true;
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/*
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* Found the item. Mark it as dead, if it isn't already.
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* Since this happens while holding a buffer lock possibly in
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* shared mode, it's possible that multiple processes attempt
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* to do this simultaneously, leading to multiple full-page
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* images being sent to WAL (if wal_log_hints or data checksums
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* are enabled), which is undesirable.
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*/
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if (!ItemIdIsDead(iid))
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{
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ItemIdMarkDead(iid);
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killedsomething= true;
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}
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break;/* out of inner search loop */
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}
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offnum=OffsetNumberNext(offnum);

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