Do not allow Unique nodes to be scanned backwards. The code claimed that it

would work, but in fact it didn't return the same rows when moving backwards
as when moving forwards.  This would have no visible effect in a DISTINCT
query (at least assuming the column datatypes use a strong definition of
equality), but it gave entirely wrong answers for DISTINCT ON queries.

diff --git a/src/backend/executor/execAmi.c b/src/backend/executor/execAmi.c
index ca7797a37ed0ac23a27a6499204eb933d7fa6a05..6de11be5b1a5f6b75ab08cee1d1535348c20bd2d 100644 (file)
--- a/src/backend/executor/execAmi.c
+++ b/src/backend/executor/execAmi.c
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *     $Header: /cvsroot/pgsql/src/backend/executor/execAmi.c,v 1.75.4.2 2004/03/02 18:56:28 tgl Exp $
+ *     $Header: /cvsroot/pgsql/src/backend/executor/execAmi.c,v 1.75.4.3 2008/08/05 21:29:01 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -338,9 +338,6 @@ ExecSupportsBackwardScan(Plan *node)
                case T_Sort:
                        return true;
 
-               case T_Unique:
-                       return ExecSupportsBackwardScan(outerPlan(node));
-
                case T_Limit:
                        return ExecSupportsBackwardScan(outerPlan(node));