diff --git a/src/backend/optimizer/README b/src/backend/optimizer/README
index a0df388993b525962093a0a26ebc4405decb2832..de04bfa81e46082062070342eeffa0e96b817874 100644
--- a/src/backend/optimizer/README
+++ b/src/backend/optimizer/README
@@ -467,7 +467,10 @@ operator is mergejoinable, so there is no way for a volatile expression to
 get into EquivalenceClasses otherwise.  Aggregates are disallowed in WHERE
 altogether, so will never be found in a mergejoinable clause.)  This is just
 a convenience to maintain a uniform PathKey representation: such an
-EquivalenceClass will never be merged with any other.
+EquivalenceClass will never be merged with any other.  Note in particular
+that a single-item EquivalenceClass {a.x} is *not* meant to imply an
+assertion that a.x = a.x; the practical effect of this is that a.x could
+be NULL.
 
 An EquivalenceClass also contains a list of btree opfamily OIDs, which
 determines what the equalities it represents actually "mean".  All the
diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
index 13ed31a5c96e9d94daefd208b38309f799e6f0dd..56e93c2c41c6e3b8051b4a36f0b447054a1e6578 100644
--- a/src/backend/optimizer/path/equivclass.c
+++ b/src/backend/optimizer/path/equivclass.c
@@ -10,7 +10,7 @@
  * 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 $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.9.2.3 2009/09/29 01:21:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -113,6 +113,19 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 	item1_relids = restrictinfo->left_relids;
 	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.
 	 */
@@ -151,13 +164,10 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 	 *
 	 * 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.)
+	 * 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;
diff --git a/src/test/regress/expected/select.out b/src/test/regress/expected/select.out
index 2936f0306aba92ea92d6f12a59c66a01b195d785..6465317231aa4c9aa299466e4bccab92cc139565 100644
--- a/src/test/regress/expected/select.out
+++ b/src/test/regress/expected/select.out
@@ -768,3 +768,19 @@ select sillysrf(-1) order by 1;
 (4 rows)
 
 drop function sillysrf(int);
+-- X = X isn't a no-op, it's effectively X IS NOT NULL assuming = is strict
+-- (see bug #5084)
+select * from (values (2),(null),(1)) v(k) where k = k order by k;
+ k 
+---
+ 1
+ 2
+(2 rows)
+
+select * from (values (2),(null),(1)) v(k) where k = k;
+ k 
+---
+ 2
+ 1
+(2 rows)
+
diff --git a/src/test/regress/sql/select.sql b/src/test/regress/sql/select.sql
index bb1fc9b9347d0e9db40550aa9cd01a4721f1b984..3bda45a2fbb83782be0f9452d3db12742944459a 100644
--- a/src/test/regress/sql/select.sql
+++ b/src/test/regress/sql/select.sql
@@ -202,3 +202,8 @@ select sillysrf(42);
 select sillysrf(-1) order by 1;
 
 drop function sillysrf(int);
+
+-- X = X isn't a no-op, it's effectively X IS NOT NULL assuming = is strict
+-- (see bug #5084)
+select * from (values (2),(null),(1)) v(k) where k = k order by k;
+select * from (values (2),(null),(1)) v(k) where k = k;