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;
    }