Fix logic in lazy vacuum to decide if it's worth trying to truncate the heap.

If the table was smaller than REL_TRUNCATE_FRACTION (= 16) pages, we always
tried to acquire AccessExclusiveLock on it even if there was no empty pages
at the end.

Report by Simon Riggs. Back-patch all the way to 7.4.

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index e809b0e56fbd5ea98ed14dd83c0433e19837bf44..a7a9065f1031a88b6163e67444971ef7b9b6265f 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -33,7 +33,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.50.4.6 2007/09/24 03:53:12 alvherre Exp $
+ *   $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.50.4.7 2009/01/06 14:56:03 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -176,8 +176,9 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt)
     * number of pages.  Otherwise, the time taken isn't worth it.
     */
    possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;
-   if (possibly_freeable >= REL_TRUNCATE_MINIMUM ||
-    possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION)
+   if (possibly_freeable > 0 &&
+       (possibly_freeable >= REL_TRUNCATE_MINIMUM ||
+        possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION))
        lazy_truncate_heap(onerel, vacrelstats);
 
    /* Update shared free space map with final free space info */