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/backend/optimizer/README

@@ -372,10 +372,10 @@ generated during the optimization process are marked with their sort order
 
 It is also possible to avoid an explicit sort step to implement a user's
 ORDER BY clause if the final path has the right ordering already, so the
-sort ordering is of interest even at the top level.  query_planner() will
+sort ordering is of interest even at the top level.  grouping_planner() will
 look for the cheapest path with a sort order matching the desired order,
-and grouping_planner() will compare its cost to the cost of using the
-cheapest-overall path and doing an explicit sort.
+then compare its cost to the cost of using the cheapest-overall path and
+doing an explicit sort on that.
 
 When we are generating paths for a particular RelOptInfo, we discard a path
 if it is more expensive than another known path that has the same or better

src/backend/optimizer/plan/planagg.c

@@ -316,6 +316,7 @@ find_minmax_aggs_walker(Node *node, List **context)
 		Assert(aggref->agglevelsup == 0);
 		if (list_length(aggref->args) != 1)
 			return true;		/* it couldn't be MIN/MAX */
+
 		/*
 		 * ORDER BY is usually irrelevant for MIN/MAX, but it can change the
 		 * outcome if the aggsortop's operator class recognizes non-identical
@@ -329,6 +330,7 @@ find_minmax_aggs_walker(Node *node, List **context)
 		 */
 		if (aggref->aggorder != NIL)
 			return true;
+
 		/*
 		 * We might implement the optimization when a FILTER clause is present
 		 * by adding the filter to the quals of the generated subquery.
@@ -399,9 +401,8 @@ build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
 	TargetEntry *tle;
 	NullTest   *ntest;
 	SortGroupClause *sortcl;
-	Path	   *cheapest_path;
+	RelOptInfo *final_rel;
 	Path	   *sorted_path;
-	double		dNumGroups;
 	Cost		path_cost;
 	double		path_fraction;
 
@@ -470,25 +471,28 @@ build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
 	 * Generate the best paths for this query, telling query_planner that we
 	 * have LIMIT 1.
 	 */
-	query_planner(subroot, parse->targetList, 1.0, 1.0,
-				  minmax_qp_callback, NULL,
-				  &cheapest_path, &sorted_path, &dNumGroups);
+	subroot->tuple_fraction = 1.0;
+	subroot->limit_tuples = 1.0;
+
+	final_rel = query_planner(subroot, parse->targetList,
+							  minmax_qp_callback, NULL);
 
 	/*
-	 * Fail if no presorted path.  However, if query_planner determines that
-	 * the presorted path is also the cheapest, it will set sorted_path to
-	 * NULL ... don't be fooled.  (This is kind of a pain here, but it
-	 * simplifies life for grouping_planner, so leave it be.)
+	 * Get the best presorted path, that being the one that's cheapest for
+	 * fetching just one row.  If there's no such path, fail.
 	 */
+	if (final_rel->rows > 1.0)
+		path_fraction = 1.0 / final_rel->rows;
+	else
+		path_fraction = 1.0;
+
+	sorted_path =
+		get_cheapest_fractional_path_for_pathkeys(final_rel->pathlist,
+												  subroot->query_pathkeys,
+												  NULL,
+												  path_fraction);
 	if (!sorted_path)
-	{
-		if (cheapest_path &&
-			pathkeys_contained_in(subroot->sort_pathkeys,
-								  cheapest_path->pathkeys))
-			sorted_path = cheapest_path;
-		else
-			return false;
-	}
+		return false;
 
 	/*
 	 * Determine cost to get just the first row of the presorted path.
@@ -496,11 +500,6 @@ build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
 	 * Note: cost calculation here should match
 	 * compare_fractional_path_costs().
 	 */
-	if (sorted_path->parent->rows > 1.0)
-		path_fraction = 1.0 / sorted_path->parent->rows;
-	else
-		path_fraction = 1.0;
-
 	path_cost = sorted_path->startup_cost +
 		path_fraction * (sorted_path->total_cost - sorted_path->startup_cost);
 

src/backend/optimizer/prep/prepjointree.c

@@ -1333,15 +1333,16 @@ is_simple_subquery(Query *subquery, RangeTblEntry *rte,
 		return false;
 
 	/*
-	 * Hack: don't try to pull up a subquery with an empty jointree.
-	 * query_planner() will correctly generate a Result plan for a jointree
-	 * that's totally empty, but I don't think the right things happen if an
-	 * empty FromExpr appears lower down in a jointree.  It would pose a
-	 * problem for the PlaceHolderVar mechanism too, since we'd have no way to
-	 * identify where to evaluate a PHV coming out of the subquery. Not worth
-	 * working hard on this, just to collapse SubqueryScan/Result into Result;
-	 * especially since the SubqueryScan can often be optimized away by
-	 * setrefs.c anyway.
+	 * Don't pull up a subquery with an empty jointree.  query_planner() will
+	 * correctly generate a Result plan for a jointree that's totally empty,
+	 * but we can't cope with an empty FromExpr appearing lower down in a
+	 * jointree: we identify join rels via baserelid sets, so we couldn't
+	 * distinguish a join containing such a FromExpr from one without it.
+	 * This would for example break the PlaceHolderVar mechanism, since we'd
+	 * have no way to identify where to evaluate a PHV coming out of the
+	 * subquery.  Not worth working hard on this, just to collapse
+	 * SubqueryScan/Result into Result; especially since the SubqueryScan can
+	 * often be optimized away by setrefs.c anyway.
 	 */
 	if (subquery->jointree->fromlist == NIL)
 		return false;

src/backend/optimizer/util/relnode.c

@@ -676,6 +676,36 @@ subbuild_joinrel_joinlist(RelOptInfo *joinrel,
 }
 
 
+/*
+ * build_empty_join_rel
+ *		Build a dummy join relation describing an empty set of base rels.
+ *
+ * This is used for queries with empty FROM clauses, such as "SELECT 2+2" or
+ * "INSERT INTO foo VALUES(...)".  We don't try very hard to make the empty
+ * joinrel completely valid, since no real planning will be done with it ---
+ * we just need it to carry a simple Result path out of query_planner().
+ */
+RelOptInfo *
+build_empty_join_rel(PlannerInfo *root)
+{
+	RelOptInfo *joinrel;
+
+	/* The dummy join relation should be the only one ... */
+	Assert(root->join_rel_list == NIL);
+
+	joinrel = makeNode(RelOptInfo);
+	joinrel->reloptkind = RELOPT_JOINREL;
+	joinrel->relids = NULL;		/* empty set */
+	joinrel->rows = 1;			/* we produce one row for such cases */
+	joinrel->width = 0;			/* it contains no Vars */
+	joinrel->rtekind = RTE_JOIN;
+
+	root->join_rel_list = lappend(root->join_rel_list, joinrel);
+
+	return joinrel;
+}
+
+
 /*
  * find_childrel_appendrelinfo
  *		Get the AppendRelInfo associated with an appendrel child rel.

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