表结构：

+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| id          | int(11)  | NO   | PRI | NULL    | auto_increment |
| total       | int(11)  | YES  |     | NULL    |                |
| thedatetime | datetime | YES  | MUL | NULL    |                |
+-------------+----------+------+-----+---------+----------------+

总行数： 137967

mysql> explain select * from out where thedatetime <= NOW();
+----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | out         | ALL  | thedatetime   | NULL | NULL    | NULL | 137967 | Using where |
+----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+

带有更多表联接的真实查询要更长得多，关键是，我无法使表使用datetime索引。如果要选择特定日期之前的所有数据，这对我来说将很难。但是，我注意到，如果选择较小的数据子集，则可以使MySQL使用索引。

mysql> explain select * from out where thedatetime <= '2008-01-01';
+----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+
| id | select_type | table       | type  | possible_keys | key         | key_len | ref  | rows  | Extra       |
+----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+
|  1 | SIMPLE      | out         | range | thedatetime   | thedatetime | 9       | NULL | 15826 | Using where |
+----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+

mysql> select count(*) from out where thedatetime <= '2008-01-01';
+----------+
| count(*) |
+----------+
|    15990 |
+----------+

那么，无论我放置什么日期，该如何确保MySQL将使用索引？