Function pathkeys_useful_for_ordering

src/backend/optimizer/path/pathkeys.c:2452–2467

* pathkeys_useful_for_ordering * Count the number of pathkeys that are useful for meeting the * query's requested output ordering. * * Because we the have the possibility of incremental sort, a prefix list of * keys is potentially useful for improving the performance of the requested * ordering. Thus we return 0, if no valuable keys are found, or the number * of leading keys shared by the

 * of leading keys shared by the list and the requested ordering..
 */
static int
pathkeys_useful_for_ordering(PlannerInfo *root, List *pathkeys)
{
	int			n_common_pathkeys;

	if (root->query_pathkeys == NIL)
		return 0;				/* no special ordering requested */

	if (pathkeys == NIL)
		return 0;				/* unordered path */

	(void) pathkeys_count_contained_in(root->query_pathkeys, pathkeys,
									   &n_common_pathkeys);

	return n_common_pathkeys;
}

/*
 * truncate_useless_pathkeys