MySql: Optimizing Subquery with index_merge - mysql

Question

I want to lists all users and the status of the newsletter-subscribtion. Since someone doesn't need to be a user if subscribed to the newsletter, I'm doing something like:
SELECT user.id, user.email,
(SELECT newsletter.status FROM newsletter
WHERE newsletter.email=user.email OR newsletter.user = user.id) AS status
FROM user
WHERE...
Indexes are user.id, user.email, newsletter.email, newsletter.user.
The OR makes the query incredible slow. I found here Union as sub query MySQL that you can do an "index merge" which will speed-up the query. But I'm not sure how to force MySQL to do an index merge in my case. Any ideas?
Added: 2nd row of explain doesn't use a key.
DROP TABLE if exists user;
DROP TABLE if exists newsletter;
create table user (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`email` varchar(255) DEFAULT NULL, INDEX email(email)
) ENGINE=InnoDB;
create table newsletter (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`status` enum('subscribed','unsubscribed') DEFAULT NULL,
`email` varchar(255) DEFAULT NULL, INDEX email(email),
`user` int(10) unsigned DEFAULT NULL, INDEX user(user)
) ENGINE=InnoDB;
EXPLAIN SELECT user.id, user.email,
(SELECT newsletter.status FROM newsletter
WHERE newsletter.email=user.email OR newsletter.user = user.id) AS status
FROM user;
+----+--------------------+------------+------------+-------+---------------+-------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+------------+------------+-------+---------------+-------+---------+------+------+----------+------------------------------------------------+
| 1 | PRIMARY | user | NULL | index | NULL | email | 768 | NULL | 1 | 100.00 | Using index |
| 2 | DEPENDENT SUBQUERY | newsletter | NULL | ALL | email,user | NULL | NULL | NULL | 1 | 100.00 | Range checked for each record (index map: 0x6) |
+----+--------------------+------------+------------+-------+---------------+-------+---------+------+------+----------+------------------------------------------------+
EXPLAIN SELECT user.id, user.email, newsletter.status
FROM user
LEFT JOIN newsletter ON (newsletter.email=user.email OR newsletter.user = user.id);
+----+-------------+------------+------------+-------+---------------+-------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+-------+---------+------+------+----------+------------------------------------------------+
| 1 | SIMPLE | user | NULL | index | NULL | email | 768 | NULL | 1 | 100.00 | Using index |
| 1 | SIMPLE | newsletter | NULL | ALL | email,user | NULL | NULL | NULL | 1 | 100.00 | Range checked for each record (index map: 0x6) |
+----+-------------+------------+------------+-------+---------------+-------+---------+------+------+----------+------------------------------------------------+

Answer 1

Unless you have switched off index_merge, MySQL will use it if it thinks it will be of benefit. But I find it kicks in less often than one might expect, and even when it does, it's not very helpful — the query is still a lot slower than using an index in the conventional way.
The typical solution for these types of queries is to do a UNION of two simpler queries.
explain select u.id, u.email, n.status
from user u left join newsletter n on u.email=n.email
union
select u.id, u.email, n.status
from user u left join newsletter n on u.id=n.user;
+----+--------------+------------+-------+---------------+-------+---------+--------------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+---------------+-------+---------+--------------+------+-----------------+
| 1 | PRIMARY | u | index | NULL | email | 403 | NULL | 1 | Using index |
| 1 | PRIMARY | n | ref | email | email | 403 | test.u.email | 1 | NULL |
| 2 | UNION | u | index | NULL | email | 403 | NULL | 1 | Using index |
| 2 | UNION | n | ref | user | user | 5 | test.u.id | 1 | NULL |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+-------+---------------+-------+---------+--------------+------+-----------------+
You'd be better off fixing your data so you only have to join on the user id, not on the email.