Function set_cheapest

src/backend/optimizer/util/pathnode.c:268–386

* set_cheapest * Find the minimum-cost paths from among a relation's paths, * and save them in the rel's cheapest-path fields. * * cheapest_total_path is normally the cheapest-total-cost unparameterized * path; but if there are no unparameterized paths, we assign it to be the * best (cheapest least-parameterized) parameterized path. However, only * unparameterized paths are considered

 * list for the rel node.
 */
void
set_cheapest(RelOptInfo *parent_rel)
{
	Path	   *cheapest_startup_path;
	Path	   *cheapest_total_path;
	Path	   *best_param_path;
	List	   *parameterized_paths;
	ListCell   *p;

	Assert(IsA(parent_rel, RelOptInfo));

	if (parent_rel->pathlist == NIL)
		elog(ERROR, "could not devise a query plan for the given query");

	cheapest_startup_path = cheapest_total_path = best_param_path = NULL;
	parameterized_paths = NIL;

	foreach(p, parent_rel->pathlist)
	{
		Path	   *path = (Path *) lfirst(p);
		int			cmp;

		if (path->param_info)
		{
			/* Parameterized path, so add it to parameterized_paths */
			parameterized_paths = lappend(parameterized_paths, path);

			/*
			 * If we have an unparameterized cheapest-total, we no longer care
			 * about finding the best parameterized path, so move on.
			 */
			if (cheapest_total_path)
				continue;

			/*
			 * Otherwise, track the best parameterized path, which is the one
			 * with least total cost among those of the minimum
			 * parameterization.
			 */
			if (best_param_path == NULL)
				best_param_path = path;
			else
			{
				switch (bms_subset_compare(PATH_REQ_OUTER(path),
										   PATH_REQ_OUTER(best_param_path)))
				{
					case BMS_EQUAL:
						/* keep the cheaper one */
						if (compare_path_costs(path, best_param_path,
											   TOTAL_COST) < 0)
							best_param_path = path;
						break;
					case BMS_SUBSET1:
						/* new path is less-parameterized */
						best_param_path = path;
						break;
					case BMS_SUBSET2:
						/* old path is less-parameterized, keep it */