postgres.git / summary / log / commit / refs

commit    9ced0134a4b0e9bd76e978259edc3540c6669883
Author:   Simon Riggs <simon@2ndQuadrant.com>
Date:     Fri Jun 01 11:59:54 2012 +0000
First released in: 9.2.0, 8.3.20, 8.4.13, 9.0.9, 9.1.5

    Avoid early reuse of btree pages, causing incorrect query results.
    When we allowed read-only transactions to skip assigning XIDs
    we introduced the possibility that a fully deleted btree page
    could be reused. This broke the index link sequence which could
    then lead to indexscans silently returning fewer rows than would
    have been correct. The actual incidence of silent errors from
    this is thought to be very low because of the exact workload
    required and locking pre-conditions. Fix is to remove pages only
    if index page opaque->btpo.xact precedes RecentGlobalXmin.
    
    Noah Misch, reviewed and backpatched by Simon Riggs


 src/backend/access/nbtree/README    | 10 ++++++----
 src/backend/access/nbtree/nbtpage.c |  2 +-
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README
index af19525cdc3..32cdba1eb69 100644
--- a/src/backend/access/nbtree/README
+++ b/src/backend/access/nbtree/README
@@ -258,13 +258,15 @@ we need to be sure we don't miss or re-scan any items.
 
 A deleted page can only be reclaimed once there is no scan or search that
 has a reference to it; until then, it must stay in place with its
-right-link undisturbed.  We implement this by waiting until all
-transactions that were running at the time of deletion are dead; which is
+right-link undisturbed.  We implement this by waiting until all active
+snapshots and registered snapshots as of the deletion are gone; which is
 overly strong, but is simple to implement within Postgres.  When marked
 dead, a deleted page is labeled with the next-transaction counter value.
 VACUUM can reclaim the page for re-use when this transaction number is
-older than the oldest open transaction.  (NOTE: VACUUM FULL can reclaim
-such pages immediately.)
+older than RecentGlobalXmin.  As collateral damage, this implementation
+also waits for running XIDs with no snapshots and for snapshots taken
+until the next transaction to allocate an XID commits.
+(NOTE: VACUUM FULL can reclaim such pages immediately.)
 
 Reclaiming a page doesn't actually change its state on disk --- we simply
 record it in the shared-memory free space map, from which it will be
diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c
index a89e3ba84f4..0e37a8a0af9 100644
--- a/src/backend/access/nbtree/nbtpage.c
+++ b/src/backend/access/nbtree/nbtpage.c
@@ -633,7 +633,7 @@ _bt_page_recyclable(Page page)
 	 */
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	if (P_ISDELETED(opaque) &&
-		TransactionIdPrecedesOrEquals(opaque->btpo.xact, RecentXmin))
+		TransactionIdPrecedes(opaque->btpo.xact, RecentGlobalXmin))
 		return true;
 	return false;
 }


[parent: 485e12fb090a]