问一个关于mysql order by的例子

mysql high performance上有下面一段话，不太明白为啥会better？
mysql> SELECT film_id, description FROM sakila.film ORDER BY title LIMIT 50, 5;
If the table is very large, this query is better written as follows:
mysql> SELECT film.film_id, film.description
  -> FROM sakila.film
  -> INNER JOIN (
  -> SELECT film_id FROM sakila.film
  -> ORDER BY title LIMIT 50, 5
  -> ) AS lim USING(film_id);

SELECT film_id FROM sakila.film
  -> ORDER BY title LIMIT 50, 5

这个只是产生了一个5条记录的结果集，然后再和sakila.film做连接，这样总记录集会很小。