postgres_fdw: Fix cost estimation for aggregate pushdown.

In commit 7012b132d0, which added support for aggregate pushdown in
postgres_fdw, the expense of evaluating the final scan/join target
computed by make_group_input_target() was not accounted for at all in
costing aggregate pushdown paths with local statistics.  The right fix
for this would be to have a separate upper stage to adjust the final
scan/join relation (see comments for apply_scanjoin_target_to_paths());
but for now, fix by adding the tlist eval cost when costing aggregate
pushdown paths with local statistics.

Apply this to HEAD only to avoid destabilizing existing plan choices.

Author: Etsuro Fujita
Reviewed-By: Antonin Houska
Discussion: https://postgr.es/m/5C66A056.60007%40lab.ntt.co.jp

contrib/postgres_fdw/postgres_fdw.c

@@ -2842,12 +2842,18 @@ estimate_path_cost_size(PlannerInfo *root,
 		}
 		else if (IS_UPPER_REL(foreignrel))
 		{
+			RelOptInfo *outerrel = fpinfo->outerrel;
 			PgFdwRelationInfo *ofpinfo;
 			AggClauseCosts aggcosts;
 			double		input_rows;
 			int			numGroupCols;
 			double		numGroups = 1;
 
+			/* The upper relation should have its outer relation set */
+			Assert(outerrel);
+			/* and that outer relation should have its reltarget set */
+			Assert(outerrel->reltarget);
+
 			/*
 			 * This cost model is mixture of costing done for sorted and
 			 * hashed aggregates in cost_agg().  We are not sure which
@@ -2856,7 +2862,7 @@ estimate_path_cost_size(PlannerInfo *root,
 			 * and all finalization and run cost are added in total_cost.
 			 */
 
-			ofpinfo = (PgFdwRelationInfo *) fpinfo->outerrel->fdw_private;
+			ofpinfo = (PgFdwRelationInfo *) outerrel->fdw_private;
 
 			/* Get rows and width from input rel */
 			input_rows = ofpinfo->rows;
@@ -2909,11 +2915,13 @@ estimate_path_cost_size(PlannerInfo *root,
 
 			/*-----
 			 * Startup cost includes:
-			 *	  1. Startup cost for underneath input relation
+			 *	  1. Startup cost for underneath input relation, adjusted for
+			 *	     tlist replacement by apply_scanjoin_target_to_paths()
 			 *	  2. Cost of performing aggregation, per cost_agg()
 			 *-----
 			 */
 			startup_cost = ofpinfo->rel_startup_cost;
+			startup_cost += outerrel->reltarget->cost.startup;
 			startup_cost += aggcosts.transCost.startup;
 			startup_cost += aggcosts.transCost.per_tuple * input_rows;
 			startup_cost += aggcosts.finalCost.startup;
@@ -2921,11 +2929,13 @@ estimate_path_cost_size(PlannerInfo *root,
 
 			/*-----
 			 * Run time cost includes:
-			 *	  1. Run time cost of underneath input relation
+			 *	  1. Run time cost of underneath input relation, adjusted for
+			 *	     tlist replacement by apply_scanjoin_target_to_paths()
 			 *	  2. Run time cost of performing aggregation, per cost_agg()
 			 *-----
 			 */
 			run_cost = ofpinfo->rel_total_cost - ofpinfo->rel_startup_cost;
+			run_cost += outerrel->reltarget->cost.per_tuple * input_rows;
 			run_cost += aggcosts.finalCost.per_tuple * numGroups;
 			run_cost += cpu_tuple_cost * numGroups;
 

src/backend/optimizer/plan/planner.c

@@ -7113,6 +7113,10 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
 	 * confused in createplan.c if they don't agree.  We must do this now so
 	 * that any append paths made in the next part will use the correct
 	 * pathtarget (cf. create_append_path).
+	 *
+	 * Note that this is also necessary if GetForeignUpperPaths() gets called
+	 * on the final scan/join relation or on any of its children, since the
+	 * FDW might look at the rel's target to create ForeignPaths.
 	 */
 	rel->reltarget = llast_node(PathTarget, scanjoin_targets);