Apply my original fix for Taiki Yamaguchi's bug report about DISTINCT MAX().

Add some regression tests for plausible failures in this area.

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 2008/01/09 20:42:27 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.10 2008/03/31 16:59:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1638,6 +1638,44 @@ add_child_rel_equivalences(PlannerInfo *root,
 }
 
 
+/*
+ * mutate_eclass_expressions
+ *	  Apply an expression tree mutator to all expressions stored in
+ *	  equivalence classes.
+ *
+ * This is a bit of a hack ... it's currently needed only by planagg.c,
+ * which needs to do a global search-and-replace of MIN/MAX Aggrefs
+ * after eclasses are already set up.  Without changing the eclasses too,
+ * subsequent matching of ORDER BY clauses would fail.
+ *
+ * Note that we assume the mutation won't affect relation membership or any
+ * other properties we keep track of (which is a bit bogus, but by the time
+ * planagg.c runs, it no longer matters).  Also we must be called in the
+ * main planner memory context.
+ */
+void
+mutate_eclass_expressions(PlannerInfo *root,
+						  Node *(*mutator) (),
+						  void *context)
+{
+	ListCell   *lc1;
+
+	foreach(lc1, root->eq_classes)
+	{
+		EquivalenceClass *cur_ec = (EquivalenceClass *) lfirst(lc1);
+		ListCell   *lc2;
+
+		foreach(lc2, cur_ec->ec_members)
+		{
+			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);
+
+			cur_em->em_expr = (Expr *)
+				mutator((Node *) cur_em->em_expr, context);
+		}
+	}
+}
+
+
 /*
  * find_eclass_clauses_for_index_join
  *	  Create joinclauses usable for a nestloop-with-inner-indexscan

src/backend/optimizer/plan/planagg.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planagg.c,v 1.36 2008/01/01 19:45:50 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planagg.c,v 1.37 2008/03/31 16:59:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -187,6 +187,18 @@ optimize_minmax_aggregates(PlannerInfo *root, List *tlist, Path *best_path)
 	hqual = replace_aggs_with_params_mutator(parse->havingQual,
 											 &aggs_list);
 
+	/*
+	 * We have to replace Aggrefs with Params in equivalence classes too,
+	 * else ORDER BY or DISTINCT on an optimized aggregate will fail.
+	 *
+	 * Note: at some point it might become necessary to mutate other
+	 * data structures too, such as the query's sortClause or distinctClause.
+	 * Right now, those won't be examined after this point.
+	 */
+	mutate_eclass_expressions(root,
+							  replace_aggs_with_params_mutator,
+							  &aggs_list);
+
 	/*
 	 * Generate the output plan --- basically just a Result
 	 */