How to optimize this heavy MySQL query? - mysql

Question

I need to optimize a MySQL query which takes a lot of time to load.
Here it is :
SELECT
p.id,
UNIX_TIMESTAMP(p.last_answer_date) AS last_answer_date_timestamp,
p.sender_id,
p.recipient_id,
p.is_read_sender,
p.last_answer_user_id,
p.is_read_recipient,
(SELECT m.read FROM pm_message m WHERE m.conv_id = p.id AND m.user_id != $user_id ORDER BY m.date DESC LIMIT 1) AS read_status,
(SELECT m.content FROM pm_message m WHERE m.conv_id = p.id ORDER BY m.date DESC LIMIT 1) AS last_message,
(SELECT u.username FROM user u WHERE (u.id = p.sender_id OR u.id = p.recipient_id) AND u.id != $user_id LIMIT 1) AS from_username,
(SELECT u.id FROM user u WHERE (u.id = p.sender_id OR u.id = p.recipient_id) AND u.id != $user_id LIMIT 1) AS from_userid,
(SELECT ui.gender FROM user_info ui WHERE (ui.user_id = p.sender_id OR ui.user_id = p.recipient_id) AND ui.user_id != $user_id LIMIT 1) AS from_gender,
(SELECT ph.thumb_url FROM photo ph, user_info ui WHERE ui.main_photo = ph.id AND (ph.user_id = p.sender_id OR ph.user_id = p.recipient_id) AND ph.user_id != $user_id LIMIT 1) AS from_thumb_url
FROM pm_conv p
WHERE p.sender_id = $user_id OR p.recipient_id = $user_id
ORDER BY p.last_answer_date DESC LIMIT 25;
This query gets me the result I want but it's really slow... And I think that the nested selects is the reason why this query is so slow.
Here are the tables structures for this query :
CREATE TABLE IF NOT EXISTS `photo` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`url` varchar(255) DEFAULT NULL,
`thumb_url` varchar(255) DEFAULT NULL,
`user_id` int(11) NOT NULL,
`date` datetime NOT NULL,
`status` int(11) NOT NULL,
`votes` int(11) DEFAULT '0',
`comments` int(11) DEFAULT '0',
`views` int(11) DEFAULT '0',
`text` text,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;
CREATE TABLE IF NOT EXISTS `pm_conv` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`date` datetime NOT NULL,
`sender_id` int(11) NOT NULL,
`recipient_id` int(11) NOT NULL,
`last_answer_date` datetime NOT NULL,
`nb_messages` int(11) NOT NULL,
`is_read_sender` int(11) NOT NULL,
`is_read_recipient` int(11) NOT NULL DEFAULT '0',
`last_answer_user_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `recipient_id` (`recipient_id`),
KEY `sender_id` (`sender_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;
CREATE TABLE IF NOT EXISTS `pm_message` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`date` datetime NOT NULL,
`content` text NOT NULL,
`user_id` int(11) NOT NULL,
`conv_id` int(11) NOT NULL,
`read` int(11) DEFAULT '0',
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
KEY `conv_id` (`conv_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;
CREATE TABLE IF NOT EXISTS `user` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`encrypt_id` varchar(255) DEFAULT NULL,
`register_date` datetime DEFAULT NULL,
`last_login_date` datetime DEFAULT NULL,
`username` varchar(255) DEFAULT NULL,
`password` varchar(255) DEFAULT NULL,
`email` varchar(255) DEFAULT NULL,
`banned` int(11) DEFAULT NULL,
`banned_reason` text,
`first_step_form` int(11) DEFAULT '0',
`status` int(11) DEFAULT NULL,
`valid_snapchat` int(11) DEFAULT '0',
`introduced_forum` int(11) DEFAULT '0',
`referer` varchar(255) DEFAULT NULL,
`allow_social_featuring` int(11) DEFAULT NULL,
`rank` int(11) DEFAULT NULL,
`fb_id` bigint(20) DEFAULT NULL,
`rate_app_status` int(11) DEFAULT NULL,
`last_activity_date` datetime DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;
CREATE TABLE IF NOT EXISTS `user_info` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`gender` int(11) DEFAULT NULL,
`birthday` date DEFAULT NULL,
`about` text,
`main_photo` int(11) DEFAULT NULL,
`country` varchar(100) DEFAULT NULL,
`city` varchar(100) DEFAULT NULL,
`relation_type` varchar(30) DEFAULT NULL,
`user_id` int(11) DEFAULT NULL,
`fb_link` varchar(255) DEFAULT NULL,
`twitter_link` varchar(255) DEFAULT NULL,
`youtube_link` varchar(255) DEFAULT NULL,
`instagram_link` varchar(255) DEFAULT NULL,
`app_pref_forum` int(11) DEFAULT NULL,
`app_pref_pm` int(11) DEFAULT NULL,
`app_pref_snapchat_request` int(11) DEFAULT NULL,
`browse_invisibly` int(11) DEFAULT '0',
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
KEY `main_photo` (`main_photo`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;
Can someone help me to optimize this heavy query?
Thanks!

Answer 1

you can see in the explain plan some tables are being accessed by inefficient indexes. try to calculate statistics on all the tables to see if it changes something (using analyze table).

Answer 2

You can join user table in order to get username and id at once instead of having two subqueries, probably you can do the same with pm_message, but it's a little trickier since subqueries have different conditions.
I would also combine user and user_info tables into, as I can see they have one-to-one relation, so it doesn't makes sense to store this data in different tables. This would allow you to get rid off 4th subquery and simplify the 5th one.
In some cases it is better to perform several queries instead of one with subqueries.