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 b3ebc9b3d2c9e7518094b68c80d014841a201bf8..198bb92686f653c09ccd564748fae7b078801f6c 100644 (file)
--- a/src/backend/executor/nodeIndexscan.c
+++ b/src/backend/executor/nodeIndexscan.c
@@ -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;