Trying to reduce mysql query, why is 'ref' NULL?

Question

Why does lean_users show NULL in the ref column? This causes my query to use a temporary table and a filesort later (when I've added more joins)...

14:45:21 (60) > EXPLAIN select * from users u inner join lean_users lu on u.id = lu.user_id;
+----+-------------+-------+--------+---------------+---------+---------+----------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref            | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+----------------+------+-------------+
|  1 | SIMPLE      | lu    | index  | PRIMARY       | PRIMARY | 4       | NULL           |  358 | Using index | 
|  1 | SIMPLE      | u     | eq_ref | PRIMARY       | PRIMARY | 4       | nwa.lu.user_id |    1 |             | 
+----+-------------+-------+--------+---------------+---------+---------+----------------+------+-------------+

users table

14:45:24 (61) > show create table users;
+-------+-----------------------------------------------------------------------------+
| Table | Create Table                                                                |
+-------+-----------------------------------------------------------------------------+
| users | CREATE TABLE `users` (
  `id` int(11) NOT NULL auto_increment,
  `email` varchar(255) default NULL,
  `first_name` varchar(50) NOT NULL,
  `last_name` varchar(50) NOT NULL,
  `address1` varchar(255) NOT NULL,
  `address2` varchar(255) default NULL,
  `city` varchar(25) NOT NULL,
  `state` mediumint(9) default NULL,
  `zip` varchar(10) NOT NULL,
  `phone` varchar(20) default NULL,
  `country` smallint(6) NOT NULL,
  `username` varchar(10) NOT NULL,
  `password` varchar(50) default NULL,
  `cdate` datetime NOT NULL,
  `last_used` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
  `level` varchar(25) default 'user',
  PRIMARY KEY  (`id`),
) ENGINE=InnoDB AUTO_INCREMENT=38076 DEFAULT CHARSET=utf8 | 
+-------+-----------------------------------------------------------------------------+

lean_users table

14:45:40 (62) > show create table lean_users;
+-------------+-----------------------------------------------------------------------------+
| Table       | Create Table                                                                |
+-------------+-----------------------------------------------------------------------------+
| lean_users  | CREATE TABLE `lean_users` (
  `user_id` int(11) NOT NULL,
  PRIMARY KEY  (`user_id`),
  CONSTRAINT `lean_users_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8                                                        | 
+-------------+-----------------------------------------------------------------------------+

Answer 1

Why does lean_users show NULL in the ref column?

Because this table is leading in the join and you don't filter on any indexed fields.

This means that each record should be read and evaluated.