Function build_join_pathkeys

src/backend/optimizer/path/pathkeys.c:1410–1429

* build_join_pathkeys * Build the path keys for a join relation constructed by mergejoin or * nestloop join. This is normally the same as the outer path's keys. * * EXCEPTION: in a FULL or RIGHT join, we cannot treat the result as * having the outer path's path keys, because null lefthand rows may be * inserted at random points. It must be treated as unsorted. * * We truncate

 * Returns the list of new path keys.
 */
List *
build_join_pathkeys(PlannerInfo *root,
					RelOptInfo *joinrel,
					JoinType jointype,
					List *outer_pathkeys)
{
	if (jointype == JOIN_FULL || jointype == JOIN_RIGHT)
		return NIL;

	/*
	 * This used to be quite a complex bit of code, but now that all pathkey
	 * sublists start out life canonicalized, we don't have to do a darn thing
	 * here!
	 *
	 * We do, however, need to truncate the pathkeys list, since it may
	 * contain pathkeys that were useful for forming this joinrel but are
	 * uninteresting to higher levels.
	 */
	return truncate_useless_pathkeys(root, joinrel, outer_pathkeys);
}


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