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

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
index 2db4015504ed69664bb4e00f2674af693cd021e8..59f54ee5cb42c75cbc1143a4f3016a9806c5b826 100644 (file)
--- 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;