Set correct cost data in Gather node added by force_parallel_mode.

We were just leaving the cost fields zeroes, which produces obviously bogus
output with force_parallel_mode = on.  With force_parallel_mode = regress,
the zeroes are hidden, but I wonder if they wouldn't still confuse add-on
code such as auto_explain.

src/backend/optimizer/plan/planner.c

@@ -343,7 +343,22 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 		gather->num_workers = 1;
 		gather->single_copy = true;
 		gather->invisible = (force_parallel_mode == FORCE_PARALLEL_REGRESS);
+
+		/*
+		 * Ideally we'd use cost_gather here, but setting up dummy path data
+		 * to satisfy it doesn't seem much cleaner than knowing what it does.
+		 */
+		gather->plan.startup_cost = top_plan->startup_cost +
+			parallel_setup_cost;
+		gather->plan.total_cost = top_plan->total_cost +
+			parallel_setup_cost + parallel_tuple_cost * top_plan->plan_rows;
+		gather->plan.plan_rows = top_plan->plan_rows;
+		gather->plan.plan_width = top_plan->plan_width;
+		gather->plan.parallel_aware = false;
+
+		/* use parallel mode for parallel plans. */
 		root->glob->parallelModeNeeded = true;
+
 		top_plan = &gather->plan;
 	}