Add bound check before bsearch() for performance

petere · petere · commit bbaf315309ed · 2021-03-10T15:19:37.000+01:00
In the current lazy vacuum implementation, some index AMs such as btree indexes call lazy_tid_reaped() for each index tuple during ambulkdelete to check if the index tuple points to the (collected) garbage tuple. In that function, we simply call bsearch(), but we should be able to know the result without bsearch() if the index tuple points to the heap tuple that is out of range of the collected garbage tuples. Therefore, add a simple bound check before resorting to bsearch(). Testing has shown that this can give significant performance benefits. Author: Masahiko Sawada <masahiko.sawada@2ndquadrant.com> Discussion: https://www.postgresql.org/message-id/flat/CA+fd4k76j8jKzJzcx8UqEugvayaMSnQz0iLUt_XgBp-_-bd22A@mail.gmail.com
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
@@ -61,6 +61,7 @@
 #include "access/visibilitymap.h"
 #include "access/xact.h"
 #include "access/xlog.h"
+#include "catalog/index.h"
 #include "catalog/storage.h"
 #include "commands/dbcommands.h"
 #include "commands/progress.h"
@@ -2923,8 +2924,24 @@ static bool
 lazy_tid_reaped(ItemPointer itemptr, void *state)
 {
 	LVDeadTuples *dead_tuples = (LVDeadTuples *) state;
+	int64		litem,
+				ritem,
+				item;
 	ItemPointer res;
 
+	litem = itemptr_encode(&dead_tuples->itemptrs[0]);
+	ritem = itemptr_encode(&dead_tuples->itemptrs[dead_tuples->num_tuples - 1]);
+	item = itemptr_encode(itemptr);
+
+	/*
+	 * Doing a simple bound check before bsearch() is useful to avoid the
+	 * extra cost of bsearch(), especially if dead tuples on the heap are
+	 * concentrated in a certain range.  Since this function is called for
+	 * every index tuple, it pays to be really fast.
+	 */
+	if (item < litem || item > ritem)
+		return false;
+
 	res = (ItemPointer) bsearch((void *) itemptr,
 								(void *) dead_tuples->itemptrs,
 								dead_tuples->num_tuples,
