Function pathkeys_count_contained_in

src/backend/optimizer/path/pathkeys.c:671–718

* pathkeys_count_contained_in * Same as pathkeys_contained_in, but also sets length of longest * common prefix of keys1 and keys2. */

 *    common prefix of keys1 and keys2.
 */
bool
pathkeys_count_contained_in(List *keys1, List *keys2, int *n_common)
{
	int			n = 0;
	ListCell   *key1,
			   *key2;

	/*
	 * See if we can avoiding looping through both lists. This optimization
	 * gains us several percent in planning time in a worst-case test.
	 */
	if (keys1 == keys2)
	{
		*n_common = list_length(keys1);
		return true;
	}
	else if (keys1 == NIL)
	{
		*n_common = 0;
		return true;
	}
	else if (keys2 == NIL)
	{
		*n_common = 0;
		return false;
	}

	/*
	 * If both lists are non-empty, iterate through both to find out how many
	 * items are shared.
	 */
	forboth(key1, keys1, key2, keys2)
	{
		PathKey    *pathkey1 = (PathKey *) lfirst(key1);
		PathKey    *pathkey2 = (PathKey *) lfirst(key2);

		if (pathkey1 != pathkey2)
		{
			*n_common = n;
			return false;
		}
		n++;
	}

	/* If we ended with a null value, then we've processed the whole list. */
	*n_common = n;
	return (key1 == NULL);
}

/*
 * get_cheapest_path_for_pathkeys