diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 65aacb99e2..6e58d561e8 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 1.342.2.5 2008/02/11 19:14:38 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.342.2.6 2008/09/11 14:01:16 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -764,14 +764,12 @@ vac_update_datfrozenxid(void)
 	bool		dirty = false;
 
 	/*
-	 * Initialize the "min" calculation with RecentGlobalXmin.  Any
-	 * not-yet-committed pg_class entries for new tables must have
-	 * relfrozenxid at least this high, because any other open xact must have
-	 * RecentXmin >= its PGPROC.xmin >= our RecentGlobalXmin; see
-	 * AddNewRelationTuple().  So we cannot produce a wrong minimum by
-	 * starting with this.
+	 * Initialize the "min" calculation with GetOldestXmin, which is a
+	 * reasonable approximation to the minimum relfrozenxid for not-yet-
+	 * committed pg_class entries for new tables; see AddNewRelationTuple().
+	 * Se we cannot produce a wrong minimum by starting with this.
 	 */
-	newFrozenXid = RecentGlobalXmin;
+	newFrozenXid = GetOldestXmin(true, true);
 
 	/*
 	 * We must seqscan pg_class to find the minimum Xid, because there is no
@@ -965,18 +963,16 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind)
 	/* Begin a transaction for vacuuming this relation */
 	StartTransactionCommand();
 
-	if (vacstmt->full)
-	{
-		/* functions in indexes may want a snapshot set */
-		ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
-	}
-	else
+	/*
+	 * Functions in indexes may want a snapshot set. Also, setting
+	 * a snapshot ensures that RecentGlobalXmin is kept truly recent.
+	 */
+	ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+
+	if (!vacstmt->full)
 	{
 		/*
-		 * During a lazy VACUUM we do not run any user-supplied functions, and
-		 * so it should be safe to not create a transaction snapshot.
-		 *
-		 * We can furthermore set the inVacuum flag, which lets other
+		 * During a lazy VACUUM we can set the inVacuum flag, which lets other
 		 * concurrent VACUUMs know that they can ignore this one while
 		 * determining their OldestXmin.  (The reason we don't set inVacuum
 		 * during a full VACUUM is exactly that we may have to run user-
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 82532a196b..15226b92bc 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.172 2006/11/05 22:42:09 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.172.2.1 2008/09/11 14:01:16 alvherre Exp $
  *
  *
  *-------------------------------------------------------------------------
@@ -42,6 +42,7 @@
 #include "utils/guc.h"
 #include "utils/portal.h"
 #include "utils/relcache.h"
+#include "utils/tqual.h"
 #include "utils/syscache.h"
 #include "pgstat.h"
 
@@ -380,10 +381,15 @@ InitPostgres(const char *dbname, const char *username)
 	on_shmem_exit(ShutdownPostgres, 0);
 
 	/*
-	 * Start a new transaction here before first access to db
+	 * Start a new transaction here before first access to db, and get a
+	 * snapshot.  We don't have a use for the snapshot itself, but we're
+	 * interested in the secondary effect that it sets RecentGlobalXmin.
 	 */
 	if (!bootstrap)
+	{
 		StartTransactionCommand();
+		(void) GetTransactionSnapshot();
+	}
 
 	/*
 	 * Now that we have a transaction, we can take locks.  Take a writer's