What is the best way to optimze this query with indexes? - mysql

Question

I have a table with about 30 million records which I need to perform queries upon. From my reading, I thought that a composite index using leftmost prefixing with all the fields I need to select would be the correct way to do it, but when I run an explain on the query, it's not even using the index.
This is the query:
select distinct email FROM my_table
WHERE `customer_id` IN(278,428,186,40,208,247,59,79,376,73,38,52,68,227)
AND `company_id` = 4
AND `active` = 1
AND `date` > '2012-04-15';
The explain looks like this
+----+-------------+--------+-------+---------------+-------+---------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+-------+---------+------+----------+-------------+
| 1 | SIMPLE | emails | index | customer_id | email | 772 | NULL | 29296705 | Using where |
+----+-------------+--------+-------+---------------+-------+---------+------+----------+-------------+
These are the fields
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`email` varchar(255) NOT NULL DEFAULT '',
`customer_id` int(10) unsigned DEFAULT NULL,
`company_id` int(10) unsigned NOT NULL,
`active` tinyint(1) unsigned NOT NULL DEFAULT '1',
`date` date DEFAULT NULL
Indexes looks like this
PRIMARY KEY (`id`),
UNIQUE KEY `email` (`email`,`customer_id`),
KEY `customer_id` (`customer_id`,`company_id`,`active`,`date`)
I'm not quite sure what the best way to optimize this is.

Answer 1

MySQL is often fussy about IN on the left side of the index. Try one query for each customer_id and see if that's using your index. You can use the UNION syntax to join them together The other possibility is that MySQL figures it's faster to sift through everything for 10% of rows than to try to use indexes for them.