No index chosen by the optimizer

MariaDB [test]> EXPLAIN SELECT * FROM testtbl WHERE id_nam=0;

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

| id   | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | testtbl | ALL  | id_nam        | NULL | NULL    | NULL | 198  | Using where |

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.000 sec)