Force optimiser to use AJ paths.

To check the feature we need good covering of tests. Force AJ whenever
possible by drastically reducing startup and total costs of OTHER_JOINREL,
generated by the AJ code.

Author: danolivo

src/backend/optimizer/path/joinrels.c

@@ -1874,10 +1874,11 @@ try_asymmetric_partitionwise_join(PlannerInfo *root,
 											 nappinfos, appinfos)));
 
 		/* And make paths for the child join */
+		force_path = true;
 		populate_joinrel_with_paths(root, outer_child, inner_rel,
 									child_joinrel, child_sjinfo,
 									child_restrictlist);
-
+		force_path = false;
 		pfree(appinfos);
 
 		/*

src/backend/optimizer/util/pathnode.c

@@ -46,6 +46,8 @@ typedef enum
  */
 #define STD_FUZZ_FACTOR 1.01
 
+bool force_path = false;
+
 static List *translate_sub_tlist(List *tlist, int relid);
 static int	append_total_cost_compare(const ListCell *a, const ListCell *b);
 static int	append_startup_cost_compare(const ListCell *a, const ListCell *b);
@@ -425,6 +427,12 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 	List	   *new_path_pathkeys;
 	ListCell   *p1;
 
+	if (force_path)
+	{
+		new_path->startup_cost = 0.0001;
+		new_path->total_cost = 0.0001;
+	}
+
 	/*
 	 * This is a convenient place to check for query cancel --- no part of the
 	 * planner goes very long without calling add_path().

src/include/optimizer/pathnode.h

@@ -18,6 +18,8 @@
 #include "nodes/pathnodes.h"
 
 
+extern bool force_path;
+
 /*
  * prototypes for pathnode.c
  */