postgres_fdw: Account for tlist eval costs in estimate_path_cost_size().

Previously, estimate_path_cost_size() didn't account for tlist eval
costs, except when costing a foreign-grouping path using local
statistics, but such costs should be accounted for when costing that path
using remote estimates, because some of the tlist expressions might be
evaluated locally.  Also, such costs should be accounted for in the case
of a foreign-scan or foreign-join path, because the tlist might contain
PlaceHolderVars, which postgres_fdw currently evaluates locally.

This also fixes an oversight in my commit f8f6e446.

Like that commit, apply this to HEAD only to avoid destabilizing existing
plan choices.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/5BFD3EAD.2060301%40lab.ntt.co.jp

contrib/postgres_fdw/postgres_fdw.c

@@ -2499,6 +2499,9 @@ estimate_path_cost_size(PlannerInfo *root,
 	Cost		total_cost;
 	Cost		cpu_per_tuple;
 
+	/* Make sure the core code has set up the relation's reltarget */
+	Assert(foreignrel->reltarget);
+
 	/*
 	 * If the table or the server is configured to use remote estimates,
 	 * connect to the foreign server and execute EXPLAIN to estimate the
@@ -2576,6 +2579,24 @@ estimate_path_cost_size(PlannerInfo *root,
 		cost_qual_eval(&local_cost, local_param_join_conds, root);
 		startup_cost += local_cost.startup;
 		total_cost += local_cost.per_tuple * retrieved_rows;
+
+		/*
+		 * Add in tlist eval cost for each output row.  In case of an
+		 * aggregate, some of the tlist expressions such as grouping
+		 * expressions will be evaluated remotely, so adjust the costs.
+		 */
+		startup_cost += foreignrel->reltarget->cost.startup;
+		total_cost += foreignrel->reltarget->cost.startup;
+		total_cost += foreignrel->reltarget->cost.per_tuple * rows;
+		if (IS_UPPER_REL(foreignrel))
+		{
+			QualCost	tlist_cost;
+
+			cost_qual_eval(&tlist_cost, fdw_scan_tlist, root);
+			startup_cost -= tlist_cost.startup;
+			total_cost -= tlist_cost.startup;
+			total_cost -= tlist_cost.per_tuple * rows;
+		}
 	}
 	else
 	{
@@ -2674,19 +2695,19 @@ estimate_path_cost_size(PlannerInfo *root,
 			nrows = clamp_row_est(nrows * fpinfo->joinclause_sel);
 			run_cost += nrows * remote_conds_cost.per_tuple;
 			run_cost += fpinfo->local_conds_cost.per_tuple * retrieved_rows;
+
+			/* Add in tlist eval cost for each output row */
+			startup_cost += foreignrel->reltarget->cost.startup;
+			run_cost += foreignrel->reltarget->cost.per_tuple * rows;
 		}
 		else if (IS_UPPER_REL(foreignrel))
 		{
 			PgFdwRelationInfo *ofpinfo;
-			PathTarget *ptarget = foreignrel->reltarget;
 			AggClauseCosts aggcosts;
 			double		input_rows;
 			int			numGroupCols;
 			double		numGroups = 1;
 
-			/* Make sure the core code set the pathtarget. */
-			Assert(ptarget != NULL);
-
 			/*
 			 * This cost model is mixture of costing done for sorted and
 			 * hashed aggregates in cost_agg().  We are not sure which
@@ -2750,26 +2771,22 @@ estimate_path_cost_size(PlannerInfo *root,
 			 * Startup cost includes:
 			 *	  1. Startup cost for underneath input relation
 			 *	  2. Cost of performing aggregation, per cost_agg()
-			 *	  3. Startup cost for PathTarget eval
 			 *-----
 			 */
 			startup_cost = ofpinfo->rel_startup_cost;
 			startup_cost += aggcosts.transCost.startup;
 			startup_cost += aggcosts.transCost.per_tuple * input_rows;
 			startup_cost += (cpu_operator_cost * numGroupCols) * input_rows;
-			startup_cost += ptarget->cost.startup;
 
 			/*-----
 			 * Run time cost includes:
 			 *	  1. Run time cost of underneath input relation
 			 *	  2. Run time cost of performing aggregation, per cost_agg()
-			 *	  3. PathTarget eval cost for each output row
 			 *-----
 			 */
 			run_cost = ofpinfo->rel_total_cost - ofpinfo->rel_startup_cost;
 			run_cost += aggcosts.finalCost * numGroups;
 			run_cost += cpu_tuple_cost * numGroups;
-			run_cost += ptarget->cost.per_tuple * numGroups;
 
 			/* Account for the eval cost of HAVING quals, if any */
 			if (root->parse->havingQual)
@@ -2784,6 +2801,10 @@ estimate_path_cost_size(PlannerInfo *root,
 				startup_cost += fpinfo->local_conds_cost.startup;
 				run_cost += fpinfo->local_conds_cost.per_tuple * retrieved_rows;
 			}
+
+			/* Add in tlist eval cost for each output row */
+			startup_cost += foreignrel->reltarget->cost.startup;
+			run_cost += foreignrel->reltarget->cost.per_tuple * rows;
 		}
 		else
 		{
@@ -2802,6 +2823,10 @@ estimate_path_cost_size(PlannerInfo *root,
 			startup_cost += foreignrel->baserestrictcost.startup;
 			cpu_per_tuple = cpu_tuple_cost + foreignrel->baserestrictcost.per_tuple;
 			run_cost += cpu_per_tuple * foreignrel->tuples;
+
+			/* Add in tlist eval cost for each output row */
+			startup_cost += foreignrel->reltarget->cost.startup;
+			run_cost += foreignrel->reltarget->cost.per_tuple * rows;
 		}
 
 		/*