Function has_useful_pathkeys

src/backend/optimizer/path/pathkeys.c:2513–2521

* has_useful_pathkeys * Detect whether the specified rel could have any pathkeys that are * useful according to truncate_useless_pathkeys(). * * This is a cheap test that lets us skip building pathkeys at all in very * simple queries. It's OK to err in the direction of returning "true" when * there really aren't any usable pathkeys, but erring in the other direction * is bad --- so keep

 * a sort are reasonably common, though, so this much work seems worthwhile.
 */
bool
has_useful_pathkeys(PlannerInfo *root, RelOptInfo *rel)
{
	if (rel->joininfo != NIL || rel->has_eclass_joins)
		return true;			/* might be able to use pathkeys for merging */
	if (root->query_pathkeys != NIL)
		return true;			/* might be able to use them for ordering */
	return false;				/* definitely useless */
}