Teach pgstat_vacuum_stat to not bother scanning pg_proc in the common case

where no function stats entries exist.  Partial response to Pavel's
observation that small VACUUM operations are noticeably slower in CVS HEAD
than 8.3.

src/backend/postmaster/pgstat.c

@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2008, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.184 2008/11/04 11:04:06 petere Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.185 2008/12/08 15:44:54 tgl Exp $
  * ----------
  */
 #include "postgres.h"
@@ -967,8 +967,12 @@ pgstat_vacuum_stat(void)
 	hash_destroy(htab);
 
 	/*
-	 * Now repeat the above steps for functions.
+	 * Now repeat the above steps for functions.  However, we needn't bother
+	 * in the common case where no function stats are being collected.
 	 */
+	if (dbentry->functions != NULL &&
+		hash_get_num_entries(dbentry->functions) > 0)
+	{
 		htab = pgstat_collect_oids(ProcedureRelationId);
 
 		pgstat_setheader(&f_msg.m_hdr, PGSTAT_MTYPE_FUNCPURGE);
@@ -1016,6 +1020,7 @@ pgstat_vacuum_stat(void)
 		}
 
 		hash_destroy(htab);
+	}
 }