mysql> EXPLAIN SELECT  `my_table_name_xxxxxxxxxxxxx`.* FROM `my_table_name_xxxxxxxxxxxxx`  WHERE `my_table_name_xxxxxxxxxxxxx`.`order_id` = 2015522 LIMIT 1;
+----+-------------+-----------------------------+------+-----------------------------------------------+------+---------+------+--------+-------------+
| id | select_type | table                       | type | possible_keys                                 | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-----------------------------+------+-----------------------------------------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | my_table_name_xxxxxxxxxxxxx | ALL  | index_my_table_name_xxxxxxxxxxxxx_on_order_id | NULL | NULL    | NULL | 280511 | Using where |
+----+-------------+-----------------------------+------+-----------------------------------------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT  `my_table_name_xxxxxxxxxxxxx`.* FROM `my_table_name_xxxxxxxxxxxxx`  WHERE `my_table_name_xxxxxxxxxxxxx`.`order_id` = '2015522' LIMIT 1;
+----+-------------+-----------------------------+------+-----------------------------------------------+-----------------------------------------------+---------+-------+------+-------------+
| id | select_type | table                       | type | possible_keys                                 | key                                           | key_len | ref   | rows | Extra       |
+----+-------------+-----------------------------+------+-----------------------------------------------+-----------------------------------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | my_table_name_xxxxxxxxxxxxx | ref  | index_my_table_name_xxxxxxxxxxxxx_on_order_id | index_my_table_name_xxxxxxxxxxxxx_on_order_id | 768     | const |    1 | Using where |
+----+-------------+-----------------------------+------+-----------------------------------------------+-----------------------------------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)