+-------+--------+---------------+---------+---------+--------------------+------+--------------------------+
| table | type   | possible_keys | key     | key_len | ref                | rows | Extra                    |
+-------+--------+---------------+---------+---------+--------------------+------+--------------------------+
| B     | ref    | PRIMARY       | PRIMARY |       2 | const              | 3007 | Using where; Using index |
| A     | eq_ref | PRIMARY       | PRIMARY |       4 | const,B.respondent |    1 | Using where; Using index |
+-------+--------+---------------+---------+---------+--------------------+------+--------------------------+