Fix set_subquery_pathlist() to copy the RTE's subquery before it gets mangled

by the planning process.  This prevents the "failed to locate grouping columns"
error recently reported by Dickson Guedes.  That happens because planning
replaces SubLinks by SubPlans in the subquery's targetlist, and exprTypmod()
is smarter about the former than the latter, causing the apparent type of
the subquery's output columns to change.  This seems to be a deficiency we
should fix in exprTypmod(), but that will be a much more invasive patch
with possible side-effects elsewhere, so I'll do that only in HEAD.

Back-patch to 8.3.  Arguably the lack of a copying step is broken/dangerous
all the way back, but in the absence of known problems I'll refrain from
making the older branches pay the extra cost.  (The reason this particular
symptom didn't appear before is that exprTypmod() wasn't smart about SubLinks
either, until 8.3.)

src/backend/optimizer/path/allpaths.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.180 2009/02/15 20:16:21 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.181 2009/03/10 20:58:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -523,6 +523,13 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	PlannerInfo *subroot;
 	List	   *pathkeys;
 
+	/*
+	 * Must copy the Query so that planning doesn't mess up the RTE contents
+	 * (really really need to fix the planner to not scribble on its input,
+	 * someday).
+	 */
+	subquery = copyObject(subquery);
+
 	/* We need a workspace for keeping track of set-op type coercions */
 	differentTypes = (bool *)
 		palloc0((list_length(subquery->targetList) + 1) * sizeof(bool));

src/test/regress/expected/subselect.out

@@ -465,3 +465,15 @@ from tc;
          3
 (2 rows)
 
+--
+-- Test case for 8.3 "failed to locate grouping columns" bug
+--
+create temp table t1 (f1 numeric(14,0), f2 varchar(30));
+select * from
+  (select distinct f1, f2, (select f2 from t1 x where x.f1 = up.f1) as fs
+   from t1 up) ss
+group by f1,f2,fs;
+ f1 | f2 | fs 
+----+----+----
+(0 rows)
+

src/test/regress/sql/subselect.sql

@@ -298,3 +298,14 @@ select
   ( select min(tb.id) from tb
     where tb.aval = (select ta.val from ta where ta.id = tc.aid) ) as min_tb_id
 from tc;
+
+--
+-- Test case for 8.3 "failed to locate grouping columns" bug
+--
+
+create temp table t1 (f1 numeric(14,0), f2 varchar(30));
+
+select * from
+  (select distinct f1, f2, (select f2 from t1 x where x.f1 = up.f1) as fs
+   from t1 up) ss
+group by f1,f2,fs;