From: Simon Riggs <simon@2ndQuadrant.com>
Date: Mon, 27 Jun 2011 21:12:09 +0000 (+0100)
Subject: Reduce impact of btree page reuse on Hot Standby by fixing off-by-1 error.
X-Git-Tag: REL9_2_BETA1~1486
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=e1cd66f74862936d84acf3008118d6094c56ad58;p=postgresql

Reduce impact of btree page reuse on Hot Standby by fixing off-by-1 error.
WAL records of type XLOG_BTREE_REUSE_PAGE were generated using a
latestRemovedXid one higher than actually needed because xid used was
page opaque->btpo.xact rather than an actually removed xid.
Noticed on an otherwise quiet system by Noah Misch.

Noah Misch and Simon Riggs
---

diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c
index 65354e6d28..4b1a2e912b 100644
--- a/src/backend/access/nbtree/nbtpage.c
+++ b/src/backend/access/nbtree/nbtpage.c
@@ -560,9 +560,19 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
 					 */
 					if (XLogStandbyInfoActive())
 					{
+						TransactionId latestRemovedXid;
+
 						BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
-						_bt_log_reuse_page(rel, blkno, opaque->btpo.xact);
+						/*
+						 * opaque->btpo.xact is the threshold value not the
+						 * value to measure conflicts against. We must retreat
+						 * by one from it to get the correct conflict xid.
+						 */
+						latestRemovedXid = opaque->btpo.xact;
+						TransactionIdRetreat(latestRemovedXid);
+
+						_bt_log_reuse_page(rel, blkno, latestRemovedXid);
 					}
 
 					/* Okay to use page.  Re-initialize and return it */