Can't understand why optimizer doesn't work properly :(

[Sergey Belov]

There is a query:

SELECT *
FROM Messages WHERE EXISTS (
SELECT 1
FROM Ask
WHERE Ask.Ask = Messages.Ask
AND Ask.DateTime < 20070710
)
LIMIT 1 , 30

Optimizer scans all `Messages` table. But all tables are indexed by necessary fields. An EXPLAIN output here:

mysql> EXPLAIN SELECT *
-> FROM Messages
-> WHERE EXISTS (
-> SELECT 1
-> FROM Ask
-> WHERE Ask.Ask = Messages.Ask
-> AND Ask.DateTime < 20070710
-> )
-> LIMIT 1 , 30;
+----+--------------------+----------+------+-----------------------------+---------+---------+--------------------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+----------+------+-----------------------------+---------+---------+--------------------+--------+-------------+
| 1 | PRIMARY | Messages | ALL | NULL | NULL | NULL | NULL | 233869 | Using where |
| 2 | DEPENDENT SUBQUERY | Ask | ref | PRIMARY,DateTime,DateTime_2 | PRIMARY | 3 | forum.Messages.Ask | 2 | Using where |
+----+--------------------+----------+------+-----------------------------+---------+---------+--------------------+--------+-------------+
2 rows in set (0.00 sec)