+----+-------------+-------+--------+---------------+---------+---------+-----------------------+-------+----------------------------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                   | rows  | Extra                                        |
+----+-------------+-------+--------+---------------+---------+---------+-----------------------+-------+----------------------------------------------+
|  1 | SIMPLE      | t     | ALL    | NULL          | NULL    | NULL    | NULL                  | 23112 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | m     | eq_ref | PRIMARY       | PRIMARY | 4       | genteya.t.ID_LAST_MSG |     1 |                                              |
+----+-------------+-------+--------+---------------+---------+---------+-----------------------+-------+----------------------------------------------+
2 rows in set (0.00 sec)

mysql> SELECT m.ID_TOPIC FROM yabbse_topics_current AS t  JOIN yabbse_messages_current AS m ON (m.ID_MSG=t.ID_LAST_MSG) WHERE t.ID_BOARD=48 ORDER BY m.posterTime DESC LIMIT 1;
+----------+
| ID_TOPIC |
+----------+
|    87622 |
+----------+
1 row in set (9.58 sec)