Innodb/MyISAM query performance differences

[Callum Bell]

Two tables, identical other than one is Innodb and the other MyISAM. 15.4 million rows in each.

Innodb:

explain select count(*) from thetable;

+----+-------------+---------+-------+---------------+---------+---------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+---------------+---------+---------+------+----------+-------------+
| 1 | SIMPLE | thetable | index | NULL | PRIMARY | 4 | NULL | 16019217 | Using index |
+----+-------------+---------+-------+---------------+---------+---------+------+----------+-------------+

The query takes more the five (5) minutes.

MyISAM:

explain select count(*) from thetable;

+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

This query executes in 0.00 seconds.

Why the difference in query plans? Can I force Innodb to query more efficiently?

Thanks,

Callum