Re-allow planner to use Merge Append to efficiently implement UNION.

This reverts commit 7204f35919,
thus restoring 66c0185a3 (Allow planner to use Merge Append to
efficiently implement UNION) as well as the follow-on commits
d5d2205c8, 3b1a7eb28, 7487044d6.

Per further discussion on pgsql-release, we wish to ship beta1 with
this feature, and patch the bug that was found just before wrap,
rather than shipping beta1 with the feature reverted.

src/include/optimizer/paths.h

@@ -173,6 +173,10 @@ extern void add_child_join_rel_equivalences(PlannerInfo *root,
 											AppendRelInfo **appinfos,
 											RelOptInfo *parent_joinrel,
 											RelOptInfo *child_joinrel);
+extern void add_setop_child_rel_equivalences(PlannerInfo *root,
+											 RelOptInfo *child_rel,
+											 List *child_tlist,
+											 List *setop_pathkeys);
 extern List *generate_implied_equalities_for_column(PlannerInfo *root,
 													RelOptInfo *rel,
 													ec_matches_callback_type callback,

src/include/optimizer/planner.h

@@ -44,7 +44,8 @@ extern PlannedStmt *standard_planner(Query *parse, const char *query_string,
 
 extern PlannerInfo *subquery_planner(PlannerGlobal *glob, Query *parse,
 									 PlannerInfo *parent_root,
-									 bool hasRecursion, double tuple_fraction);
+									 bool hasRecursion, double tuple_fraction,
+									 SetOperationStmt *setops);
 
 extern RowMarkType select_rowmark_type(RangeTblEntry *rte,
 									   LockClauseStrength strength);

src/include/optimizer/prep.h

@@ -53,6 +53,6 @@ extern void preprocess_aggrefs(PlannerInfo *root, Node *clause);
  * prototypes for prepunion.c
  */
 extern RelOptInfo *plan_set_operations(PlannerInfo *root);
-
+extern bool set_operation_ordered_results_useful(SetOperationStmt *setop);
 
 #endif							/* PREP_H */