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

hlinnaka · hlinnaka · commit 7ffe657225ad · 2009-01-06T14:55:37.000Z
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
@@ -29,7 +29,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.115 2009/01/01 17:23:40 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.116 2009/01/06 14:55:37 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -183,8 +183,9 @@ 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);
 
 	/* Vacuum the Free Space Map */

Original file line number	Diff line number	Diff line change
`@@ -29,7 +29,7 @@`
`29`	`29`	`*`
`30`	`30`	`*`
`31`	`31`	`* IDENTIFICATION`
`32`		`- * $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.115 2009/01/01 17:23:40 momjian Exp $`
	`32`	`+ * $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.116 2009/01/06 14:55:37 heikki Exp $`
`33`	`33`	`*`
`34`	`34`	`*-------------------------------------------------------------------------`
`35`	`35`	`*/`
`@@ -183,8 +183,9 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,`
`183`	`183`	`* number of pages. Otherwise, the time taken isn't worth it.`
`184`	`184`	`*/`
`185`	`185`	`possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;`
`186`		`- if (possibly_freeable >= REL_TRUNCATE_MINIMUM \|\|`
`187`		`- possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION)`
	`186`	`+ if (possibly_freeable > 0 &&`
	`187`	`+ (possibly_freeable >= REL_TRUNCATE_MINIMUM \|\|`
	`188`	`+ possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION))`
`188`	`189`	`lazy_truncate_heap(onerel, vacrelstats);`
`189`	`190`
`190`	`191`	`/* Vacuum the Free Space Map */`