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/backend/optimizer/path/allpaths.c

@@ -2633,9 +2633,8 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	Assert(root->plan_params == NIL);
 
 	/* Generate a subroot and Paths for the subquery */
-	rel->subroot = subquery_planner(root->glob, subquery,
-									root,
-									false, tuple_fraction);
+	rel->subroot = subquery_planner(root->glob, subquery, root, false,
+									tuple_fraction, NULL);
 
 	/* Isolate the params needed by this specific subplan */
 	rel->subplan_params = root->plan_params;

src/backend/optimizer/path/equivclass.c

@@ -2882,6 +2882,67 @@ add_child_join_rel_equivalences(PlannerInfo *root,
 	MemoryContextSwitchTo(oldcontext);
 }
 
+/*
+ * add_setop_child_rel_equivalences
+ *		Add equivalence members for each non-resjunk target in 'child_tlist'
+ *		to the EquivalenceClass in the corresponding setop_pathkey's pk_eclass.
+ *
+ * 'root' is the PlannerInfo belonging to the top-level set operation.
+ * 'child_rel' is the RelOptInfo of the child relation we're adding
+ * EquivalenceMembers for.
+ * 'child_tlist' is the target list for the setop child relation.  The target
+ * list expressions are what we add as EquivalenceMembers.
+ * 'setop_pathkeys' is a list of PathKeys which must contain an entry for each
+ * non-resjunk target in 'child_tlist'.
+ */
+void
+add_setop_child_rel_equivalences(PlannerInfo *root, RelOptInfo *child_rel,
+								 List *child_tlist, List *setop_pathkeys)
+{
+	ListCell   *lc;
+	ListCell   *lc2 = list_head(setop_pathkeys);
+
+	foreach(lc, child_tlist)
+	{
+		TargetEntry *tle = lfirst_node(TargetEntry, lc);
+		EquivalenceMember *parent_em;
+		PathKey    *pk;
+
+		if (tle->resjunk)
+			continue;
+
+		if (lc2 == NULL)
+			elog(ERROR, "too few pathkeys for set operation");
+
+		pk = lfirst_node(PathKey, lc2);
+		parent_em = linitial(pk->pk_eclass->ec_members);
+
+		/*
+		 * We can safely pass the parent member as the first member in the
+		 * ec_members list as this is added first in generate_union_paths,
+		 * likewise, the JoinDomain can be that of the initial member of the
+		 * Pathkey's EquivalenceClass.
+		 */
+		add_eq_member(pk->pk_eclass,
+					  tle->expr,
+					  child_rel->relids,
+					  parent_em->em_jdomain,
+					  parent_em,
+					  exprType((Node *) tle->expr));
+
+		lc2 = lnext(setop_pathkeys, lc2);
+	}
+
+	/*
+	 * transformSetOperationStmt() ensures that the targetlist never contains
+	 * any resjunk columns, so all eclasses that exist in 'root' must have
+	 * received a new member in the loop above.  Add them to the child_rel's
+	 * eclass_indexes.
+	 */
+	child_rel->eclass_indexes = bms_add_range(child_rel->eclass_indexes, 0,
+											  list_length(root->eq_classes) - 1);
+}
+
 
 /*
  * generate_implied_equalities_for_column

src/backend/optimizer/path/pathkeys.c

@@ -2191,6 +2191,22 @@ pathkeys_useful_for_grouping(PlannerInfo *root, List *pathkeys)
 	return n;
 }
 
+/*
+ * pathkeys_useful_for_setop
+ *		Count the number of leading common pathkeys root's 'setop_pathkeys' in
+ *		'pathkeys'.
+ */
+static int
+pathkeys_useful_for_setop(PlannerInfo *root, List *pathkeys)
+{
+	int			n_common_pathkeys;
+
+	(void) pathkeys_count_contained_in(root->setop_pathkeys, pathkeys,
+									   &n_common_pathkeys);
+
+	return n_common_pathkeys;
+}
+
 /*
  * truncate_useless_pathkeys
  *		Shorten the given pathkey list to just the useful pathkeys.
@@ -2208,6 +2224,9 @@ truncate_useless_pathkeys(PlannerInfo *root,
 	if (nuseful2 > nuseful)
 		nuseful = nuseful2;
 	nuseful2 = pathkeys_useful_for_grouping(root, pathkeys);
+	if (nuseful2 > nuseful)
+		nuseful = nuseful2;
+	nuseful2 = pathkeys_useful_for_setop(root, pathkeys);
 	if (nuseful2 > nuseful)
 		nuseful = nuseful2;