mysql> EXPLAIN SELECT a.name FROM users a INNER JOIN sessions b ON a.name = b.name
+----+-------------+-------+--------+------------------+------------+---------+-----------------+------+---------------------------------+
| id | select_type | table | type   | possible_keys    | key        | key_len | ref             | rows | Extra                           |
+----+-------------+-------+--------+------------------+------------+---------+-----------------+------+---------------------------------+
|  1 | SIMPLE      | b     | ALL    | NULL             | NULL       |    NULL | NULL            |    8 | Using temporary; Using filesort |
|  1 | SIMPLE      | a     | eq_ref | PRIMARY          | PRIMARY    |       3 | users.b.user_id |    1 | Using where                     |
|  1 | SIMPLE      | c     | ref    | usernummer,kombi | usernummer |       3 | users.a.id      |    2 | Using index                     |
+----+-------------+-------+--------+------------------+------------+---------+-----------------+------+---------------------------------+
3 rows in set (0.00 sec)



mysql> EXPLAIN SELECT b.name FROM sessions a INNER JOIN users b ON a.name = b.name
+----+-------------+-------+--------+------------------+------------+---------+-----------------+------+---------------------------------+
| id | select_type | table | type   | possible_keys    | key        | key_len | ref             | rows | Extra                           |
+----+-------------+-------+--------+------------------+------------+---------+-----------------+------+---------------------------------+
|  1 | SIMPLE      | a     | ALL    | NULL             | NULL       |    NULL | NULL            |    7 | Using temporary; Using filesort |
|  1 | SIMPLE      | b     | eq_ref | PRIMARY          | PRIMARY    |       3 | users.a.user_id |    1 | Using where                     |
|  1 | SIMPLE      | c     | ref    | usernummer,kombi | usernummer |       3 | users.a.user_id |    2 | Using index                     |
+----+-------------+-------+--------+------------------+------------+---------+-----------------+------+---------------------------------+
3 rows in set (0.00 sec)