Function adjust_foreign_grouping_path_cost

contrib/postgres_fdw/postgres_fdw.c:3633–3680

* Adjust the cost estimates of a foreign grouping path to include the cost of * generating properly-sorted output. */

 * generating properly-sorted output.
 */
static void
adjust_foreign_grouping_path_cost(PlannerInfo *root,
								  List *pathkeys,
								  double retrieved_rows,
								  double width,
								  double limit_tuples,
								  Cost *p_startup_cost,
								  Cost *p_run_cost)
{
	/*
	 * If the GROUP BY clause isn't sort-able, the plan chosen by the remote
	 * side is unlikely to generate properly-sorted output, so it would need
	 * an explicit sort; adjust the given costs with cost_sort().  Likewise,
	 * if the GROUP BY clause is sort-able but isn't a superset of the given
	 * pathkeys, adjust the costs with that function.  Otherwise, adjust the
	 * costs by applying the same heuristic as for the scan or join case.
	 */
	if (!grouping_is_sortable(root->parse->groupClause) ||
		!pathkeys_contained_in(pathkeys, root->group_pathkeys))
	{
		Path		sort_path;	/* dummy for result of cost_sort */

		cost_sort(&sort_path,
				  root,
				  pathkeys,
				  *p_startup_cost + *p_run_cost,
				  retrieved_rows,
				  width,
				  0.0,
				  work_mem,
				  limit_tuples);

		*p_startup_cost = sort_path.startup_cost;
		*p_run_cost = sort_path.total_cost - sort_path.startup_cost;
	}
	else
	{
		/*
		 * The default extra cost seems too large for foreign-grouping cases;
		 * add 1/4th of that default.
		 */
		double		sort_multiplier = 1.0 + (DEFAULT_FDW_SORT_MULTIPLIER
											 - 1.0) * 0.25;

		*p_startup_cost *= sort_multiplier;
		*p_run_cost *= sort_multiplier;
	}
}

/*
 * Detect whether we want to process an EquivalenceClass member.