query.sql

mysql> select count(*) from residents;
+----------+
| count(*) |
+----------+
|   601087 |
+----------+
1 row in set (0.21 sec)

mysql> select count(*) from users;
+----------+
| count(*) |
+----------+
|    48144 |
+----------+
1 row in set (0.03 sec)

mysql> explain SELECT `residents`.* FROM `residents` INNER JOIN `users` ON `users`.`id` = `residents`.`user_id` WHERE (users.id = 50713) LIMIT 1 ;
+----+-------------+-----------+-------+-------------------+-------------------+---------+-------+------+-------------+
| id | select_type | table     | type  | possible_keys     | key               | key_len | ref   | rows | Extra       |
+----+-------------+-----------+-------+-------------------+-------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | users     | const | PRIMARY           | PRIMARY           | 4       | const |    1 | Using index |
|  1 | SIMPLE      | residents | ref   | residents_by_user | residents_by_user | 5       | const |    1 | Using where |
+----+-------------+-----------+-------+-------------------+-------------------+---------+-------+------+-------------+
2 rows in set (0.00 sec)