Slow query when using GROUP BY - mysql

Question

I have the following query:
SELECT id, user_id, cookieId, text_date
FROM `_history`
WHERE text_date BETWEEN '2014-09-01' AND '2014-10-01' AND user_id = 1
GROUP BY cookieId
ORDER BY id DESC
My table schema:
CREATE TABLE `_history` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) NOT NULL,
`cookieId` varchar(50) NOT NULL,
`text_from` varchar(50) NOT NULL,
`text_body` text NOT NULL,
`text_date` datetime NOT NULL,
`aName` varchar(50) NOT NULL,
`hasArrived` enum('0','1') NOT NULL COMMENT,
`agent_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `cookieId` (`cookieId`),
KEY `user_id` (`user_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
An EXPLAN yields this:
1 SIMPLE _history ref cookieId,user_id user_id 4 const 49837 Using where; Using temporary; Using filesort
Sometimes the query takes 2 seconds and sometimes its up to 5s.
Any ideas how to make this run faster?

Answer 1

The group by does nothing at the moment so drop it.
The user_id already has an index on it, so the query and sort on it are fine.
The text_date has no index on it, adding an index on it should speed up your query.
If this query occurs often, add a composite index on both user_id and text_date.
eg.
create index idx_text_date on `_history` (text_date);
Based on the comments, the query should look like this:
SELECT cookieId, max(id), max(user_id), max(text_date)
FROM `_history`
WHERE text_date BETWEEN '2014-09-01' AND '2014-10-01'
AND user_id = 1
GROUP BY cookieId
ORDER BY id DESC
And the index should look like this:
create index idx__history_text_date_cookieId on `_history` (text_date, cookieId);

Answer 2

Create a composite index on (user_id, cookieId).
CREATE TABLE `_history` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) NOT NULL,
`cookieId` varchar(50) NOT NULL,
`text_from` varchar(50) NOT NULL,
`text_body` text NOT NULL,
`text_date` datetime NOT NULL,
`aName` varchar(50) NOT NULL,
`hasArrived` enum('0','1') NOT NULL COMMENT,
`agent_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `cookieId` (`cookieId`),
KEY `user_id_X_cookieId` (`user_id`, `cookieId`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
It will then be able to use the user_id index to find the rows, and use the cookieId suffix of that index to group them.
When you have this index, you don't need the user_id index, because a prefix of a composite index can be used as an index.