Advance multiple array keys rightmost-first instead of leftmost-first

tglsfdc · tglsfdc · commit 8e850b9159b1 · 2008-03-18T03:54:52.000Z
during a bitmap index scan.  This cannot affect the query results
(since we're just dumping the TIDs into a bitmap) but it might offer
some advantage in locality of access to the index.  Per Greg Stark.
diff --git a/src/backend/executor/nodeIndexscan.c b/src/backend/executor/nodeIndexscan.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeIndexscan.c,v 1.125 2008/01/01 19:45:49 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeIndexscan.c,v 1.126 2008/03/18 03:54:52 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -352,7 +352,13 @@ ExecIndexAdvanceArrayKeys(IndexArrayKeyInfo *arrayKeys, int numArrayKeys)
 	bool		found = false;
 	int			j;
 
-	for (j = 0; j < numArrayKeys; j++)
+	/*
+	 * Note we advance the rightmost array key most quickly, since it will
+	 * correspond to the lowest-order index column among the available
+	 * qualifications.  This is hypothesized to result in better locality
+	 * of access in the index.
+	 */
+	for (j = numArrayKeys - 1; j >= 0; j--)
 	{
 		ScanKey		scan_key = arrayKeys[j].scan_key;
 		int			next_elem = arrayKeys[j].next_elem;

Original file line number	Diff line number	Diff line change
`@@ -8,7 +8,7 @@`
`8`	`8`	`*`
`9`	`9`	`*`
`10`	`10`	`* IDENTIFICATION`
`11`		`- * $PostgreSQL: pgsql/src/backend/executor/nodeIndexscan.c,v 1.125 2008/01/01 19:45:49 momjian Exp $`
	`11`	`+ * $PostgreSQL: pgsql/src/backend/executor/nodeIndexscan.c,v 1.126 2008/03/18 03:54:52 tgl Exp $`
`12`	`12`	`*`
`13`	`13`	`*-------------------------------------------------------------------------`
`14`	`14`	`*/`
`@@ -352,7 +352,13 @@ ExecIndexAdvanceArrayKeys(IndexArrayKeyInfo *arrayKeys, int numArrayKeys)`
`352`	`352`	`bool found = false;`
`353`	`353`	`int j;`
`354`	`354`
`355`		`- for (j = 0; j < numArrayKeys; j++)`
	`355`	`+ /*`
	`356`	`+ * Note we advance the rightmost array key most quickly, since it will`
	`357`	`+ * correspond to the lowest-order index column among the available`
	`358`	`+ * qualifications. This is hypothesized to result in better locality`
	`359`	`+ * of access in the index.`
	`360`	`+ */`
	`361`	`+ for (j = numArrayKeys - 1; j >= 0; j--)`
`356`	`362`	`{`
`357`	`363`	`ScanKey scan_key = arrayKeys[j].scan_key;`
`358`	`364`	`int next_elem = arrayKeys[j].next_elem;`