Track the current XID wrap limit (or more accurately, the oldest unfrozen

XID) in checkpoint records.  This eliminates the need to recompute the value
from scratch during database startup, which is one of the two remaining
reasons for the flatfile code to exist.  It should also simplify life for
hot-standby operation.

To avoid bloating the checkpoint records unreasonably, I switched from
tracking the oldest database by name to tracking it by OID.  This turns
out to save cycles in general (everywhere but the warning-generating
paths, which we hardly care about) and also helps us deal with the case
that the oldest database got dropped instead of being vacuumed.  The prior
coding might go for a long time without updating the wrap limit in that case,
which is bad because it might result in a lot of useless autovacuum activity.

1 files changed, 25 insertions, 2 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index ed427785517..7f675a985cb 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.349 2009/08/27 07:15:41 heikki Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.350 2009/08/31 02:23:22 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -34,6 +34,7 @@
 #include "access/xlogutils.h"
 #include "catalog/catversion.h"
 #include "catalog/pg_control.h"
+#include "catalog/pg_database.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"
 #include "libpq/pqsignal.h"
@@ -4638,12 +4639,16 @@ BootStrapXLOG(void)
 	checkPoint.nextOid = FirstBootstrapObjectId;
 	checkPoint.nextMulti = FirstMultiXactId;
 	checkPoint.nextMultiOffset = 0;
+	checkPoint.oldestXid = FirstNormalTransactionId;
+	checkPoint.oldestXidDB = TemplateDbOid;
 	checkPoint.time = (pg_time_t) time(NULL);
 
 	ShmemVariableCache->nextXid = checkPoint.nextXid;
 	ShmemVariableCache->nextOid = checkPoint.nextOid;
 	ShmemVariableCache->oidCount = 0;
 	MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
+	ShmemVariableCache->oldestXid = checkPoint.oldestXid;
+	ShmemVariableCache->oldestXidDB = checkPoint.oldestXidDB;
 
 	/* Set up the XLOG page header */
 	page->xlp_magic = XLOG_PAGE_MAGIC;
@@ -5355,6 +5360,9 @@ StartupXLOG(void)
 	ereport(DEBUG1,
 			(errmsg("next MultiXactId: %u; next MultiXactOffset: %u",
 					checkPoint.nextMulti, checkPoint.nextMultiOffset)));
+	ereport(DEBUG1,
+			(errmsg("oldest unfrozen transaction ID: %u, in database %u",
+					checkPoint.oldestXid, checkPoint.oldestXidDB)));
 	if (!TransactionIdIsNormal(checkPoint.nextXid))
 		ereport(PANIC,
 				(errmsg("invalid next transaction ID")));
@@ -5363,6 +5371,8 @@ StartupXLOG(void)
 	ShmemVariableCache->nextOid = checkPoint.nextOid;
 	ShmemVariableCache->oidCount = 0;
 	MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
+	ShmemVariableCache->oldestXid = checkPoint.oldestXid;
+	ShmemVariableCache->oldestXidDB = checkPoint.oldestXidDB;
 
 	/*
 	 * We must replay WAL entries using the same TimeLineID they were created
@@ -6546,6 +6556,8 @@ CreateCheckPoint(int flags)
 	 */
 	LWLockAcquire(XidGenLock, LW_SHARED);
 	checkPoint.nextXid = ShmemVariableCache->nextXid;
+	checkPoint.oldestXid = ShmemVariableCache->oldestXid;
+	checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
 	LWLockRelease(XidGenLock);
 
 	/* Increase XID epoch if we've wrapped around since last checkpoint */
@@ -6984,6 +6996,8 @@ xlog_redo(XLogRecPtr lsn, XLogRecord *record)
 		ShmemVariableCache->oidCount = 0;
 		MultiXactSetNextMXact(checkPoint.nextMulti,
 							  checkPoint.nextMultiOffset);
+		ShmemVariableCache->oldestXid = checkPoint.oldestXid;
+		ShmemVariableCache->oldestXidDB = checkPoint.oldestXidDB;
 
 		/* ControlFile->checkPointCopy always tracks the latest ckpt XID */
 		ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
@@ -7022,6 +7036,12 @@ xlog_redo(XLogRecPtr lsn, XLogRecord *record)
 		}
 		MultiXactAdvanceNextMXact(checkPoint.nextMulti,
 								  checkPoint.nextMultiOffset);
+		if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
+								  checkPoint.oldestXid))
+		{
+			ShmemVariableCache->oldestXid = checkPoint.oldestXid;
+			ShmemVariableCache->oldestXidDB = checkPoint.oldestXidDB;
+		}
 
 		/* ControlFile->checkPointCopy always tracks the latest ckpt XID */
 		ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
@@ -7056,13 +7076,16 @@ xlog_desc(StringInfo buf, uint8 xl_info, char *rec)
 		CheckPoint *checkpoint = (CheckPoint *) rec;
 
 		appendStringInfo(buf, "checkpoint: redo %X/%X; "
-						 "tli %u; xid %u/%u; oid %u; multi %u; offset %u; %s",
+						 "tli %u; xid %u/%u; oid %u; multi %u; offset %u; "
+						 "oldest xid %u in DB %u; %s",
 						 checkpoint->redo.xlogid, checkpoint->redo.xrecoff,
 						 checkpoint->ThisTimeLineID,
 						 checkpoint->nextXidEpoch, checkpoint->nextXid,
 						 checkpoint->nextOid,
 						 checkpoint->nextMulti,
 						 checkpoint->nextMultiOffset,
+						 checkpoint->oldestXid,
+						 checkpoint->oldestXidDB,
 				 (info == XLOG_CHECKPOINT_SHUTDOWN) ? "shutdown" : "online");
 	}
 	else if (info == XLOG_NOOP)