postgrespro.git / summary / log / commit / refs

commit    242dfcbafac592a3f097ec2e4e36fe1b739f7f29
Author:   Alvaro Herrera <alvherre@alvh.no-ip.org>
Date:     Fri May 15 20:50:34 2020 +0000

    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 <masahiko.sawada@2ndquadrant.com>
    Discussion: https://postgr.es/m/CA+fd4k6PeRj2CkzapWNrERkja5G0-6D-YQiKfbukJV+qZGFZ_Q@mail.gmail.com


 src/backend/access/nbtree/nbtutils.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c
index 84b391ce03..1429ac8b63 100644
--- a/src/backend/access/nbtree/nbtutils.c
+++ b/src/backend/access/nbtree/nbtutils.c
@@ -1854,7 +1854,15 @@ _bt_killitems(IndexScanDesc scan)
 			else if (ItemPointerEquals(&ituple->t_tid, &kitem->heapTid))
 				killtuple = true;
 
-			if (killtuple)
+			/*
+			 * Mark index item 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
+			 * set to WAL (if wal_log_hints or data checksums are enabled),
+			 * which is undesirable.
+			 */
+			if (killtuple && !ItemIdIsDead(iid))
 			{
 				/* found the item/all posting list items */
 				ItemIdMarkDead(iid);


[parent: 5da14938f7bf]