mysql> explain select * from orm where oid='10000';
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | orm   | ref  | PRIMARY,oid   | PRIMARY | 302     | const |   23 | Using where |
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from orm where oid=10000;
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | orm   | ALL  | PRIMARY,oid   | NULL | NULL    | NULL | 310000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)