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, 9 insertions, 8 deletions

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 1058bd2d312..d2b3105e028 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.390 2009/08/24 02:18:31 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.391 2009/08/31 02:23:22 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -895,8 +895,9 @@ vac_update_datfrozenxid(void)
 	/*
 	 * If we were able to advance datfrozenxid, mark the flat-file copy of
 	 * pg_database for update at commit, and see if we can truncate pg_clog.
+	 * Also force update if the shared XID-wrap-limit info is stale.
 	 */
-	if (dirty)
+	if (dirty || !TransactionIdLimitIsValid())
 	{
 		database_file_update_needed();
 		vac_truncate_clog(newFrozenXid);
@@ -916,7 +917,7 @@ vac_update_datfrozenxid(void)
  *
  *		This routine is shared by full and lazy VACUUM.  Note that it's
  *		only invoked when we've managed to change our DB's datfrozenxid
- *		entry.
+ *		entry, or we found that the shared XID-wrap-limit info is stale.
  */
 static void
 vac_truncate_clog(TransactionId frozenXID)
@@ -925,11 +926,11 @@ vac_truncate_clog(TransactionId frozenXID)
 	Relation	relation;
 	HeapScanDesc scan;
 	HeapTuple	tuple;
-	NameData	oldest_datname;
+	Oid			oldest_datoid;
 	bool		frozenAlreadyWrapped = false;
 
-	/* init oldest_datname to sync with my frozenXID */
-	namestrcpy(&oldest_datname, get_database_name(MyDatabaseId));
+	/* init oldest_datoid to sync with my frozenXID */
+	oldest_datoid = MyDatabaseId;
 
 	/*
 	 * Scan pg_database to compute the minimum datfrozenxid
@@ -958,7 +959,7 @@ vac_truncate_clog(TransactionId frozenXID)
 		else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
 		{
 			frozenXID = dbform->datfrozenxid;
-			namecpy(&oldest_datname, &dbform->datname);
+			oldest_datoid = HeapTupleGetOid(tuple);
 		}
 	}
 
@@ -987,7 +988,7 @@ vac_truncate_clog(TransactionId frozenXID)
 	 * Update the wrap limit for GetNewTransactionId.  Note: this function
 	 * will also signal the postmaster for an(other) autovac cycle if needed.
 	 */
-	SetTransactionIdLimit(frozenXID, &oldest_datname);
+	SetTransactionIdLimit(frozenXID, oldest_datoid);
 }