*** pgsql/src/backend/optimizer/path/equivclass.c	2008/12/01 21:06:20	1.9.2.2
--- pgsql/src/backend/optimizer/path/equivclass.c	2009/09/29 01:21:02	1.9.2.3
***************
*** 10,16 ****
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * IDENTIFICATION
!  *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.9.2.1 2008/03/31 16:59:33 tgl Exp $
   *
   *-------------------------------------------------------------------------
   */
--- 10,16 ----
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * IDENTIFICATION
!  *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.9.2.2 2008/12/01 21:06:20 tgl Exp $
   *
   *-------------------------------------------------------------------------
   */
*************** process_equivalence(PlannerInfo *root, R
*** 114,119 ****
--- 114,132 ----
  	item2_relids = restrictinfo->right_relids;
  
  	/*
+ 	 * Reject clauses of the form X=X.  These are not as redundant as they
+ 	 * might seem at first glance: assuming the operator is strict, this is
+ 	 * really an expensive way to write X IS NOT NULL.  So we must not risk
+ 	 * just losing the clause, which would be possible if there is already
+ 	 * a single-element EquivalenceClass containing X.  The case is not
+ 	 * common enough to be worth contorting the EC machinery for, so just
+ 	 * reject the clause and let it be processed as a normal restriction
+ 	 * clause.
+ 	 */
+ 	if (equal(item1, item2))
+ 		return false;			/* X=X is not a useful equivalence */
+ 
+ 	/*
  	 * If below outer join, check for strictness, else reject.
  	 */
  	if (below_outer_join)
*************** process_equivalence(PlannerInfo *root, R
*** 151,163 ****
  	 *
  	 * 4. We find neither.	Make a new, two-entry EC.
  	 *
! 	 * Note: since all ECs are built through this process, it's impossible
! 	 * that we'd match an item in more than one existing EC.  It is possible
! 	 * to match more than once within an EC, if someone fed us something silly
! 	 * like "WHERE X=X".  (However, we can't simply discard such clauses,
! 	 * since they should fail when X is null; so we will build a 2-member EC
! 	 * to ensure the correct restriction clause gets generated.  Hence there
! 	 * is no shortcut here for item1 and item2 equal.)
  	 */
  	ec1 = ec2 = NULL;
  	em1 = em2 = NULL;
--- 164,173 ----
  	 *
  	 * 4. We find neither.	Make a new, two-entry EC.
  	 *
! 	 * Note: since all ECs are built through this process or the similar
! 	 * search in get_eclass_for_sort_expr(), it's impossible that we'd match
! 	 * an item in more than one existing nonvolatile EC.  So it's okay to stop
! 	 * at the first match.
  	 */
  	ec1 = ec2 = NULL;
  	em1 = em2 = NULL;