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 edde32d12dfd9b82eefe54cc7b40173fd9080952..967536393eec0621f37430cb886635b45b07555c 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
- *       $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.140 2005/03/27 23:53:02 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.141 2005/04/04 01:43:12 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -917,16 +917,17 @@ 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 *) linitial(mergeclauses);
                if (firstclause->left_mergescansel < 0) /* not computed yet? */
@@ -946,10 +947,14 @@ 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;
        }