MariaDB:

+------+-------------+-------+--------+---------------+---------+---------+------------------------+------+-------------+
| id   | select_type | table | type   | possible_keys | key     | key_len | ref                    | rows | Extra       |
+------+-------------+-------+--------+---------------+---------+---------+------------------------+------+-------------+
|    1 | SIMPLE      | t1    | index  | NULL          | PRIMARY | 4       | NULL                   |   20 | Using where |
|    1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | foo.t1.data_id         |    1 | Using where |
+------+-------------+-------+--------+---------------+---------+---------+------------------------+------+-------------+

Oracle MySQL:
+----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                       | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+-------------+
|  1 | SIMPLE      | t1    | index  | NULL          | PRIMARY | 4       | NULL                      |   20 | Using where |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | foo.t1.data_id            |    1 |             |
+----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+-------------+