diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 1c2384e8415a570e376c1b18dcd98096c0c22d3b..0c11d5e55b34b23ae1179f53696b476be2c44e24 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.292.2.5 2008/10/30 04:06:25 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.292.2.6 2009/05/07 11:25:28 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -6481,6 +6481,19 @@ pg_start_backup(PG_FUNCTION_ARGS)
 	XLogCtl->Insert.forcePageWrites = true;
 	LWLockRelease(WALInsertLock);
 
+	/*
+	 * Force an XLOG file switch before the checkpoint, to ensure that the WAL
+	 * segment the checkpoint is written to doesn't contain pages with old
+	 * timeline IDs. That would otherwise happen if you called
+	 * pg_start_backup() right after restoring from a PITR archive: the first
+	 * WAL segment containing the startup checkpoint has pages in the
+	 * beginning with the old timeline ID. That can cause trouble at recovery:
+	 * we won't have a history file covering the old timeline if pg_xlog
+	 * directory was not included in the base backup and the WAL archive was
+	 * cleared too before starting the backup.
+	 */
+	RequestXLogSwitch();
+
 	/* Ensure we release forcePageWrites if fail below */
 	PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) 0);
 	{