How to Optimize a Query With GROUP BY and ORDER BY - mysql

Question

I have got a POSTS table, the structure is like this:
CREATE TABLE IF NOT EXISTS `posts` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`title` varchar(255) COLLATE utf8_turkish_ci DEFAULT NULL,
`content` longtext COLLATE utf8_turkish_ci,
`excerpt` longtext COLLATE utf8_turkish_ci,
`link` longtext COLLATE utf8_turkish_ci,
`original_link` longtext COLLATE utf8_turkish_ci,
`mime_type` longtext COLLATE utf8_turkish_ci,
`language_id` int(11) DEFAULT NULL,
`user_id` int(11) DEFAULT NULL,
`site_id` int(11) DEFAULT NULL,
`type` varchar(255) COLLATE utf8_turkish_ci DEFAULT NULL,
`created_at` datetime DEFAULT NULL,
`modified_at` datetime DEFAULT NULL,
`is_deleted` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
KEY `type` (`type`),
KEY `created_at` (`created_at`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_turkish_ci AUTO_INCREMENT=52487 ;
And a USERS table, structed like this:
CREATE TABLE IF NOT EXISTS `users` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`username` varchar(255) COLLATE utf8_turkish_ci NOT NULL,
PRIMARY KEY (`id`),
KEY `username` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_turkish_ci AUTO_INCREMENT=9422 ;
I'm using this query to get latest "page, file or post" posts ordered by descending time and grouping by user to not show all latest posts from a user:
SELECT p.*, u.*
FROM posts p
LEFT JOIN users u ON p.user_id = u.id
WHERE p.type IN ('post', 'page', 'file')
GROUP BY p.user_id
ORDER BY p.created_at DESC
LIMIT 30
But it is too slow, even limited to 30 records.
now, how can i speed up this query? which columns to index or any other ideas? thanks.

Answer 1

The first thing to do is to add an index on posts.user_id (or maybe posts.user_id + posts.type). And another index on posts.created_at
UPDATE
I've just payed attention that your query grabs all fields from both tables, and posts table has 6 long text columns. So I believe you have a poor performance because mysql has to create quite a large temporary table or temp file to get all rows for satisfying your group by + order by clauses.
I think the following query should help.
SELECT u.*, p1.* FROM
users u
INNER JOIN
(
SELECT p.user_id, p.created_at, p.id FROM posts p
WHERE p.type IN ('post', 'page', 'file') GROUP by p.user_id
ORDER BY p.created_at DESC LIMIT 30
)xxx ON xxx.user_id = u.id
INNER JOIN posts p1 ON (p1.id = xxx.id)

Answer 2

In terms of indices, I would suggest creating ones on posts.type (WHERE), posts.created_at (ORDER). That should help speed up the sorting.

Answer 3

You can try this:
SELECT p., u.
FROM (SELECT * FROM posts WHERE p.type IN ('post', 'page', 'file')) p
LEFT JOIN users u ON p.user_id = u.id
GROUP BY p.user_id ORDER BY p.created_at DESC
LIMIT 30
MySQL first proccess the inner query, and with its result process the outter query with less records.

Answer 4

Try #Gabriel's answer, but with the LIMIT in the inner query.
SELECT p., u.
FROM (SELECT * FROM posts WHERE type IN ('post', 'page', 'file') ORDER BY created_at DESC LIMIT 30) p
LEFT JOIN users u ON p.user_id = u.id
ORDER BY p.created_at;