While doing the final setrefs.c pass over a plan tree, try to match up

non-Var sort/group expressions using ressortgroupref labels instead of
depending entirely on equal()-ity of the upper node's tlist expressions
to the lower node's.  This avoids emitting the wrong outputs in cases
where there are textually identical volatile sort/group expressions,
as for example
	select distinct random(),random() from generate_series(1,10);
Per report from Andrew Gierth.

Backpatch to 8.4.  Arguably this is wrong all the way back, but the only known
case where there's an observable problem is when using hash aggregation to
implement DISTINCT, which is new as of 8.4.  So for the moment I'll refrain
from backpatching further.

src/backend/optimizer/plan/setrefs.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/setrefs.c,v 1.154 2009/10/26 02:26:34 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/setrefs.c,v 1.155 2009/11/16 18:04:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -101,6 +101,10 @@ static Var *search_indexed_tlist_for_var(Var *var,
 static Var *search_indexed_tlist_for_non_var(Node *node,
 								 indexed_tlist *itlist,
 								 Index newvarno);
+static Var *search_indexed_tlist_for_sortgroupref(Node *node,
+									  Index sortgroupref,
+									  indexed_tlist *itlist,
+									  Index newvarno);
 static List *fix_join_expr(PlannerGlobal *glob,
 			  List *clauses,
 			  indexed_tlist *outer_itlist,
@@ -1197,10 +1201,25 @@ set_upper_references(PlannerGlobal *glob, Plan *plan, int rtoffset)
 		TargetEntry *tle = (TargetEntry *) lfirst(l);
 		Node	   *newexpr;
 
-		newexpr = fix_upper_expr(glob,
-								 (Node *) tle->expr,
-								 subplan_itlist,
-								 rtoffset);
+		/* If it's a non-Var sort/group item, first try to match by sortref */
+		if (tle->ressortgroupref != 0 && !IsA(tle->expr, Var))
+		{
+			newexpr = (Node *)
+				search_indexed_tlist_for_sortgroupref((Node *) tle->expr,
+													  tle->ressortgroupref,
+													  subplan_itlist,
+													  OUTER);
+			if (!newexpr)
+				newexpr = fix_upper_expr(glob,
+										 (Node *) tle->expr,
+										 subplan_itlist,
+										 rtoffset);
+		}
+		else
+			newexpr = fix_upper_expr(glob,
+									 (Node *) tle->expr,
+									 subplan_itlist,
+									 rtoffset);
 		tle = flatCopyTargetEntry(tle);
 		tle->expr = (Expr *) newexpr;
 		output_targetlist = lappend(output_targetlist, tle);
@@ -1444,6 +1463,49 @@ search_indexed_tlist_for_non_var(Node *node,
 	return NULL;				/* no match */
 }
 
+/*
+ * search_indexed_tlist_for_sortgroupref --- find a sort/group expression
+ *		(which is assumed not to be just a Var)
+ *
+ * If a match is found, return a Var constructed to reference the tlist item.
+ * If no match, return NULL.
+ *
+ * This is needed to ensure that we select the right subplan TLE in cases
+ * where there are multiple textually-equal()-but-volatile sort expressions.
+ * And it's also faster than search_indexed_tlist_for_non_var.
+ */
+static Var *
+search_indexed_tlist_for_sortgroupref(Node *node,
+									  Index sortgroupref,
+									  indexed_tlist *itlist,
+									  Index newvarno)
+{
+	ListCell   *lc;
+
+	foreach(lc, itlist->tlist)
+	{
+		TargetEntry *tle = (TargetEntry *) lfirst(lc);
+
+		/* The equal() check should be redundant, but let's be paranoid */
+		if (tle->ressortgroupref == sortgroupref &&
+			equal(node, tle->expr))
+		{
+			/* Found a matching subplan output expression */
+			Var		   *newvar;
+
+			newvar = makeVar(newvarno,
+							 tle->resno,
+							 exprType((Node *) tle->expr),
+							 exprTypmod((Node *) tle->expr),
+							 0);
+			newvar->varnoold = 0;	/* wasn't ever a plain Var */
+			newvar->varoattno = 0;
+			return newvar;
+		}
+	}
+	return NULL;				/* no match */
+}
+
 /*
  * fix_join_expr
  *	   Create a new set of targetlist entries or join qual clauses by