In cost_mergejoin, the early-exit effect should not apply to the

outer side of an outer join.  Per andrew@supernews.

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index c151ddd5f167eed5261d468fd7900780ef61c5f3..3406f9708c7d58f80c1e7fe92cca4c8aa5bf42f8 100644 (file)
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -49,7 +49,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.115.2.2 2004/04/06 18:46:25 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.115.2.3 2005/04/04 01:43:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -919,16 +919,17 @@ cost_mergejoin(MergePath *path, Query *root)
        rescanratio = 1.0 + (rescannedtuples / inner_path_rows);
 
        /*
-        * A merge join will stop as soon as it exhausts either input stream.
-        * Estimate fraction of the left and right inputs that will actually
-        * need to be scanned.  We use only the first (most significant) merge
-        * clause for this purpose.
+        * A merge join will stop as soon as it exhausts either input stream
+        * (unless it's an outer join, in which case the outer side has to be
+        * scanned all the way anyway).  Estimate fraction of the left and right
+        * inputs that will actually need to be scanned. We use only the first
+        * (most significant) merge clause for this purpose.
         *
         * Since this calculation is somewhat expensive, and will be the same for
         * all mergejoin paths associated with the merge clause, we cache the
         * results in the RestrictInfo node.
         */
-       if (mergeclauses)
+       if (mergeclauses && path->jpath.jointype != JOIN_FULL)
        {
                firstclause = (RestrictInfo *) lfirst(mergeclauses);
                if (firstclause->left_mergescansel < 0) /* not computed yet? */
@@ -948,10 +949,14 @@ cost_mergejoin(MergePath *path, Query *root)
                        outerscansel = firstclause->right_mergescansel;
                        innerscansel = firstclause->left_mergescansel;
                }
+               if (path->jpath.jointype == JOIN_LEFT)
+                       outerscansel = 1.0;
+               else if (path->jpath.jointype == JOIN_RIGHT)
+                       innerscansel = 1.0;
        }
        else
        {
-               /* cope with clauseless mergejoin */
+               /* cope with clauseless or full mergejoin */
                outerscansel = innerscansel = 1.0;
        }