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 9c56945b2fe527603448b605ae4d0bfa702feb1b..612b049997fc6ee5fdb29cafc7f6a7851cad0b3e 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -175,8 +175,9 @@ 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 */