Avoid killing btree items that are already dead

_bt_killitems marks btree items dead when a scan leaves the page where
they live, but it does so with only share lock (to improve concurrency).
This was historicall okay, since killing a dead item has no
consequences.  However, with the advent of data checksums and
wal_log_hints, this action incurs a WAL full-page-image record of the
page.  Multiple concurrent processes would write the same page several
times, leading to WAL bloat.  The probability of this happening can be
reduced by only killing items if they're not already dead, so change the
code to do that.

The problem could eliminated completely by having _bt_killitems upgrade
to exclusive lock upon seeing a killable item, but that would reduce
concurrency so it's considered a cure worse than the disease.

Backpatch all the way back to 9.5, since wal_log_hints was introduced in
9.4.

Author: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/CA+fd4k6PeRj2CkzapWNrERkja5G0-6D-YQiKfbukJV+qZGFZ_Q@mail.gmail.com

diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c
index 88d0b12f93fca1791e2691469280a55a0270d0a9..a23d693f0de7183d33392b2c74cf4d1de97795a7 100644 (file)
--- a/src/backend/access/nbtree/nbtutils.c
+++ b/src/backend/access/nbtree/nbtutils.c
@@ -1801,9 +1801,19 @@ _bt_killitems(IndexScanDesc scan)
 
            if (ItemPointerEquals(&ituple->t_tid, &kitem->heapTid))
            {
-               /* found the item */
-               ItemIdMarkDead(iid);
-               killedsomething = true;
+               /*
+                * Found the item.  Mark it as dead, if it isn't already.
+                * Since this happens while holding a buffer lock possibly in
+                * shared mode, it's possible that multiple processes attempt
+                * to do this simultaneously, leading to multiple full-page
+                * images being sent to WAL (if wal_log_hints or data checksums
+                * are enabled), which is undesirable.
+                */
+               if (!ItemIdIsDead(iid))
+               {
+                   ItemIdMarkDead(iid);
+                   killedsomething = true;
+               }
                break;          /* out of inner search loop */
            }
            offnum = OffsetNumberNext(offnum);