mysql> desc users;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | NO   |     | 0       |       |
| name  | varchar(200) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> desc accounts;
+---------+---------+------+-----+---------+-------+
| Field   | Type    | Null | Key | Default | Extra |
+---------+---------+------+-----+---------+-------+
| id      | int(11) | NO   |     | 0       |       |
| user_id | int(11) | YES  |     | NULL    |       |
| amount  | int(11) | YES  |     | NULL    |       |
+---------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> explain select u.name from users as u join accounts as a on u.id = a.user_id where a.amount > 1000;
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | u     | ALL  | NULL          | NULL | NULL    | NULL |   10 |                                |
|  1 | SIMPLE      | a     | ALL  | NULL          | NULL | NULL    | NULL |   10 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
2 rows in set (0.00 sec)

mysql> alter table users add primary key (id);
Query OK, 0 rows affected (0.26 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table accounts add primary key (id);
Query OK, 0 rows affected (0.25 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table accounts add key (user_id);
Query OK, 0 rows affected (0.21 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select u.name from users as u join accounts as a on u.id = a.user_id where a.amount > 1000;
+----+-------------+-------+------+---------------+---------+---------+-----------+------+-------------+
| id | select_type | table | type | possible_keys | key     | key_len | ref       | rows | Extra       |
+----+-------------+-------+------+---------------+---------+---------+-----------+------+-------------+
|  1 | SIMPLE      | u     | ALL  | PRIMARY       | NULL    | NULL    | NULL      |   10 |             |
|  1 | SIMPLE      | a     | ref  | user_id       | user_id | 5       | test.u.id |    1 | Using where |
+----+-------------+-------+------+---------------+---------+---------+-----------+------+-------------+
2 rows in set (0.00 sec)