Function compare_pathkeys

src/backend/optimizer/path/pathkeys.c:613–645

* compare_pathkeys * Compare two pathkeys to see if they are equivalent, and if not whether * one is "better" than the other. * * We assume the pathkeys are canonical, and so they can be checked for * equality by simple pointer comparison. */

 *	  equality by simple pointer comparison.
 */
PathKeysComparison
compare_pathkeys(List *keys1, List *keys2)
{
	ListCell   *key1,
			   *key2;

	/*
	 * Fall out quickly if we are passed two identical lists.  This mostly
	 * catches the case where both are NIL, but that's common enough to
	 * warrant the test.
	 */
	if (keys1 == keys2)
		return PATHKEYS_EQUAL;

	forboth(key1, keys1, key2, keys2)
	{
		PathKey    *pathkey1 = (PathKey *) lfirst(key1);
		PathKey    *pathkey2 = (PathKey *) lfirst(key2);

		if (pathkey1 != pathkey2)
			return PATHKEYS_DIFFERENT;	/* no need to keep looking */
	}

	/*
	 * If we reached the end of only one list, the other is longer and
	 * therefore not a subset.
	 */
	if (key1 != NULL)
		return PATHKEYS_BETTER1;	/* key1 is longer */
	if (key2 != NULL)
		return PATHKEYS_BETTER2;	/* key2 is longer */
	return PATHKEYS_EQUAL;
}

/*
 * pathkeys_contained_in