Optimisation problem: sometimes uses index, sometimes doesnt??

[Ben XO]

Hi, i have an weird optimisation problem illustrated by the following:

mysql> explain select * from doa_show_instance where start > '2005-09-07' \G
*************************** 1. row ***************************
table: doa_show_instance
type: ALL
possible_keys: start
key: NULL
key_len: NULL
ref: NULL
rows: 13295
Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from doa_show_instance where start > '2005-09-08' \G
*************************** 1. row ***************************
table: doa_show_instance
type: range
possible_keys: start
key: start
key_len: 8
ref: NULL
rows: 2310
Extra: Using where
1 row in set (0.00 sec)


(one uses index, one doesnt).

The table looks like this:

+-------------+--------------+------+-----+---------------------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------------------+----------------+
| id | int(11) | | PRI | NULL | auto_increment |
| show_id | int(11) | | MUL | 0 | |
| start | datetime | | MUL | 0000-00-00 00:00:00 | |
| length | time | | | 00:00:00 | |
| regular | tinyint(1) | | | 1 | |
| description | text | | | | |
| guests | varchar(250) | YES | | NULL | |
| highlight | int(1) | YES | | NULL | |
+-------------+--------------+------+-----+---------------------+----------------+

mysql> show index from doa_show_instance
-> ;
+-------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| doa_show_instance | 0 | PRIMARY | 1 | id | A | 13295 | NULL | NULL | | BTREE | |
| doa_show_instance | 1 | start | 1 | start | A | 6647 | NULL | NULL | | BTREE | |
| doa_show_instance | 1 | show_id | 1 | show_id | A | 172 | NULL | NULL | | BTREE | |
+-------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

any idea what gives?

thanks in advance.