Simplify query_planner's API by having it return the top-level RelOptInfo.

Formerly, query_planner returned one or possibly two Paths for the topmost
join relation, so that grouping_planner didn't see the join RelOptInfo
(at least not directly; it didn't have any hesitation about examining
cheapest_path->parent, though).  However, correct selection of the Paths
involved a significant amount of coupling between query_planner and
grouping_planner, a problem which has gotten worse over time.  It seems
best to give up on this API choice and instead return the topmost
RelOptInfo explicitly.  Then grouping_planner can pull out the Paths it
wants from the rel's path list.  In this way we can remove all knowledge
of grouping behaviors from query_planner.

The only real benefit of the old way is that in the case of an empty
FROM clause, we never made any RelOptInfos at all, just a Path.  Now
we have to gin up a dummy RelOptInfo to represent the empty FROM clause.
That's not a very big deal though.

While at it, simplify query_planner's API a bit more by having the caller
set up root->tuple_fraction and root->limit_tuples, rather than passing
those values as separate parameters.  Since query_planner no longer does
anything with either value, requiring it to fill the PlannerInfo fields
seemed pretty arbitrary.

This patch just rearranges code; it doesn't (intentionally) change any
behaviors.  Followup patches will do more interesting things.

src/include/optimizer/pathnode.h

@@ -142,6 +142,7 @@ extern RelOptInfo *build_join_rel(PlannerInfo *root,
 			   RelOptInfo *inner_rel,
 			   SpecialJoinInfo *sjinfo,
 			   List **restrictlist_ptr);
+extern RelOptInfo *build_empty_join_rel(PlannerInfo *root);
 extern AppendRelInfo *find_childrel_appendrelinfo(PlannerInfo *root,
 							RelOptInfo *rel);
 extern ParamPathInfo *get_baserel_parampathinfo(PlannerInfo *root,

src/include/optimizer/planmain.h

@@ -27,11 +27,8 @@ typedef void (*query_pathkeys_callback) (PlannerInfo *root, void *extra);
 /*
  * prototypes for plan/planmain.c
  */
-extern void query_planner(PlannerInfo *root, List *tlist,
-			  double tuple_fraction, double limit_tuples,
-			  query_pathkeys_callback qp_callback, void *qp_extra,
-			  Path **cheapest_path, Path **sorted_path,
-			  double *num_groups);
+extern RelOptInfo *query_planner(PlannerInfo *root, List *tlist,
+			  query_pathkeys_callback qp_callback, void *qp_extra);
 
 /*
  * prototypes for plan/planagg.c