Fix generation of MergeAppend plans for optimized min/max on expressions.

Before jamming a desired targetlist into a plan node, one really ought to
make sure the plan node can handle projections, and insert a buffering
Result plan node if not.  planagg.c forgot to do this, which is a hangover
from the days when it only dealt with IndexScan plan types.  MergeAppend
doesn't project though, not to mention that it gets unhappy if you remove
its possibly-resjunk sort columns.  The code accidentally failed to fail
for cases in which the min/max argument was a simple Var, because the new
targetlist would be equivalent to the original "flat" tlist anyway.
For any more complex case, it's been broken since 9.1 where we introduced
the ability to optimize min/max using MergeAppend, as reported by Raphael
Bauduin.  Fix by duplicating the logic from grouping_planner that decides
whether we need a Result node.

In 9.2 and 9.1, this requires back-porting the tlist_same_exprs() function
introduced in commit 4387cf95, else we'd
uselessly add a Result node in cases that worked before.  It's rather
tempting to back-patch that whole commit so that we can avoid extra Result
nodes in mainline cases too; but I'll refrain, since that code hasn't
really seen all that much field testing yet.

src/backend/optimizer/plan/planagg.c

@@ -39,6 +39,7 @@
 #include "optimizer/planmain.h"
 #include "optimizer/planner.h"
 #include "optimizer/subselect.h"
+#include "optimizer/tlist.h"
 #include "parser/parsetree.h"
 #include "parser/parse_clause.h"
 #include "utils/lsyscache.h"
@@ -545,7 +546,27 @@ make_agg_subplan(PlannerInfo *root, MinMaxAggInfo *mminfo)
 	 */
 	plan = create_plan(subroot, mminfo->path);
 
+	/*
+	 * If the top-level plan node is one that cannot do expression evaluation
+	 * and its existing target list isn't already what we need, we must insert
+	 * a Result node to project the desired tlist.
+	 */
+	if (!is_projection_capable_plan(plan) &&
+		!tlist_same_exprs(subparse->targetList, plan->targetlist))
+	{
+		plan = (Plan *) make_result(subroot,
+									subparse->targetList,
+									NULL,
+									plan);
+	}
+	else
+	{
+		/*
+		 * Otherwise, just replace the subplan's flat tlist with the desired
+		 * tlist.
+		 */
 		plan->targetlist = subparse->targetList;
+	}
 
 	plan = (Plan *) make_limit(plan,
 							   subparse->limitOffset,

src/test/regress/expected/inherit.out

@@ -1207,6 +1207,28 @@ select * from matest0 order by 1-id;
   1 | Test 1
 (6 rows)
 
+explain (verbose, costs off) select min(1-id) from matest0;
+               QUERY PLAN               
+----------------------------------------
+ Aggregate
+   Output: min((1 - matest0.id))
+   ->  Append
+         ->  Seq Scan on public.matest0
+               Output: matest0.id
+         ->  Seq Scan on public.matest1
+               Output: matest1.id
+         ->  Seq Scan on public.matest2
+               Output: matest2.id
+         ->  Seq Scan on public.matest3
+               Output: matest3.id
+(11 rows)
+
+select min(1-id) from matest0;
+ min 
+-----
+  -5
+(1 row)
+
 reset enable_indexscan;
 set enable_seqscan = off;  -- plan with fewest seqscans should be merge
 explain (verbose, costs off) select * from matest0 order by 1-id;
@@ -1238,6 +1260,42 @@ select * from matest0 order by 1-id;
   1 | Test 1
 (6 rows)
 
+explain (verbose, costs off) select min(1-id) from matest0;
+                                QUERY PLAN                                
+--------------------------------------------------------------------------
+ Result
+   Output: $0
+   InitPlan 1 (returns $0)
+     ->  Limit
+           Output: ((1 - matest0.id))
+           ->  Result
+                 Output: ((1 - matest0.id))
+                 ->  Merge Append
+                       Sort Key: ((1 - matest0.id))
+                       ->  Index Scan using matest0i on public.matest0
+                             Output: matest0.id, (1 - matest0.id)
+                             Index Cond: ((1 - matest0.id) IS NOT NULL)
+                       ->  Index Scan using matest1i on public.matest1
+                             Output: matest1.id, (1 - matest1.id)
+                             Index Cond: ((1 - matest1.id) IS NOT NULL)
+                       ->  Sort
+                             Output: matest2.id, ((1 - matest2.id))
+                             Sort Key: ((1 - matest2.id))
+                             ->  Bitmap Heap Scan on public.matest2
+                                   Output: matest2.id, (1 - matest2.id)
+                                   Filter: ((1 - matest2.id) IS NOT NULL)
+                                   ->  Bitmap Index Scan on matest2_pkey
+                       ->  Index Scan using matest3i on public.matest3
+                             Output: matest3.id, (1 - matest3.id)
+                             Index Cond: ((1 - matest3.id) IS NOT NULL)
+(25 rows)
+
+select min(1-id) from matest0;
+ min 
+-----
+  -5
+(1 row)
+
 reset enable_seqscan;
 drop table matest0 cascade;
 NOTICE:  drop cascades to 3 other objects

src/test/regress/sql/inherit.sql

@@ -382,11 +382,15 @@ insert into matest3 (name) values ('Test 6');
 set enable_indexscan = off;  -- force use of seqscan/sort, so no merge
 explain (verbose, costs off) select * from matest0 order by 1-id;
 select * from matest0 order by 1-id;
+explain (verbose, costs off) select min(1-id) from matest0;
+select min(1-id) from matest0;
 reset enable_indexscan;
 
 set enable_seqscan = off;  -- plan with fewest seqscans should be merge
 explain (verbose, costs off) select * from matest0 order by 1-id;
 select * from matest0 order by 1-id;
+explain (verbose, costs off) select min(1-id) from matest0;
+select min(1-id) from matest0;
 reset enable_seqscan;
 
 drop table matest0 cascade;