GROUP BY got confused if there were multiple equal() GROUP BY items.

This bug has been latent since 7.0 or maybe even further back, but it
was only exposed when parse_clause.c stopped suppressing duplicate
items (see its rev 1.96 of 18-Aug-02).

src/backend/optimizer/plan/planner.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/planner.c,v 1.151 2003/03/10 03:53:50 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/planner.c,v 1.152 2003/03/13 16:58:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1489,13 +1489,14 @@ make_groupsortplan(Query *parse,
 				   Plan *subplan)
 {
 	List	   *sort_tlist = new_unsorted_tlist(subplan->targetlist);
+	int			grpno = 0;
 	int			keyno = 0;
 	List	   *gl;
 
 	foreach(gl, groupClause)
 	{
 		GroupClause *grpcl = (GroupClause *) lfirst(gl);
-		TargetEntry *te = nth(grpColIdx[keyno] - 1, sort_tlist);
+		TargetEntry *te = nth(grpColIdx[grpno] - 1, sort_tlist);
 		Resdom	   *resdom = te->resdom;
 
 		/*
@@ -1509,6 +1510,7 @@ make_groupsortplan(Query *parse,
 			resdom->reskey = ++keyno;
 			resdom->reskeyop = grpcl->sortop;
 		}
+		grpno++;
 	}
 
 	Assert(keyno > 0);