mysql> explain select id, confession from Confessions.confessions where id = (select confession_id from Confessions.scores where score > 0.5 order by rand() limit 1);
+----+----------------------+-------------+------+---------------+------+---------+------+--------+----------------------------------------------+
| id | select_type          | table       | type | possible_keys | key  | key_len | ref  | rows   | Extra                                        |
+----+----------------------+-------------+------+---------------+------+---------+------+--------+----------------------------------------------+
|  1 | PRIMARY              | confessions | ALL  | NULL          | NULL | NULL    | NULL | 212769 | Using where                                  |
|  2 | UNCACHEABLE SUBQUERY | scores      | ALL  | NULL          | NULL | NULL    | NULL |     64 | Using where; Using temporary; Using filesort |
+----+----------------------+-------------+------+---------------+------+---------+------+--------+----------------------------------------------+