mysql> EXPLAIN SELECT img_name, img_size FROM categorylinks, page, image WHERE cl_from=page_id AND cl_to = 'Astronomy' AND page_title = img_name ORDER BY img_size;
+----+-------------+---------------+--------+---------------------------------+--------------+---------+-----------------------------------+------+-----------------------------------------------------------+
| id | select_type | table         | type   | possible_keys                   | key          | key_len | ref                               | rows | Extra                                                     |
+----+-------------+---------------+--------+---------------------------------+--------------+---------+-----------------------------------+------+-----------------------------------------------------------+
|  1 | SIMPLE      | categorylinks | ref    | cl_from,cl_timestamp,cl_sortkey | cl_timestamp | 257     | const                             |  227 | Using where; Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | page          | eq_ref | PRIMARY                         | PRIMARY      | 4       | commonswiki.categorylinks.cl_from |    1 |                                                           |
|  1 | SIMPLE      | image         | eq_ref | PRIMARY                         | PRIMARY      | 257     | commonswiki.page.page_title       |    1 |                                                           |
+----+-------------+---------------+--------+---------------------------------+--------------+---------+-----------------------------------+------+-----------------------------------------------------------+
3 rows in set (0.00 sec)

mysql> EXPLAIN SELECT img_name, img_size FROM categorylinks FORCE INDEX (cl_from), page, image WHERE cl_from=page_id AND cl_to = 'Astronomy' AND img_name = page_title ORDER BY img_size;
+----+-------------+---------------+--------+---------------+------------+---------+--------------------------------+----------+----------------------------------------------+
| id | select_type | table         | type   | possible_keys | key        | key_len | ref                            | rows     | Extra                                        |
+----+-------------+---------------+--------+---------------+------------+---------+--------------------------------+----------+----------------------------------------------+
|  1 | SIMPLE      | page          | index  | PRIMARY       | name_title | 261     | NULL                           | 19081712 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | image         | eq_ref | PRIMARY       | PRIMARY    | 257     | commonswiki.page.page_title    |        1 |                                              |
|  1 | SIMPLE      | categorylinks | eq_ref | cl_from       | cl_from    | 261     | commonswiki.page.page_id,const |        1 | Using where; Using index                     |
+----+-------------+---------------+--------+---------------+------------+---------+--------------------------------+----------+----------------------------------------------+
3 rows in set (0.00 sec)