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 c4bf719cebaae2b47d2764d42cb44e680320c4b1..d0010d336c0d118a4573f1cb74e5d8547e9c8272 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -38,7 +38,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.81.2.4 2007/09/24 03:53:02 alvherre Exp $
+ *       $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.81.2.5 2009/01/06 14:55:50 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -181,8 +181,9 @@ 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 */