Question

This query is a bit slow, and I’d like to optimize it, any ideas?

SELECT DISTINCT a.id
FROM   article a
       LEFT JOIN article_comment ac
         ON a.id = ac.article_id
       LEFT JOIN comment c
         ON ac.id = c.id
WHERE  a.id IN (SELECT a2.id
                  FROM   article_user_read aur
                         LEFT JOIN article a2
                           ON aur.article_id = a2.id
                  WHERE  c.published_date > aur.read_date
                     AND aur.user_id = 36748
                     AND aur.followed = 1)
ORDER  BY c.published_date DESC

here is the article_user_read table :

CREATE TABLE `article_user_read` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `article_id` int(11) DEFAULT NULL,
  `user_id` int(11) DEFAULT NULL,
  `read_date` datetime NOT NULL,
  `followed` tinyint(1) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `UNIQ_BBE52A0262922701A76ED395` (`article_id`,`user_id`),
  KEY `IDX_BBE52A0262922701` (`article_id`),
  KEY `IDX_BBE52A02A76ED395` (`user_id`),
  CONSTRAINT `article_user_read_ibfk_3` FOREIGN KEY (`article_id`) REFERENCES `article` (`id`),
  CONSTRAINT `article_user_read_ibfk_4` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=20193 DEFAULT CHARSET=latin1;

The other are just simple article & user table.

Answer 1

SELECT DISTINCT a.id
  FROM article AS a

  JOIN article_user_read AS aur
    ON aur.article_id = a.id

  JOIN comment AS c
    ON ac.id = c.id

  JOIN article_comment AS ac
    ON a.id = ac.article_id

 WHERE c.published_date > aur.read_date
   AND aur.user_id = 36748
   AND aur.followed = 1

 ORDER BY c.published_date DESC

It doesn’t make sense to LEFT JOIN article_user_read with article, because you’re interested in articles, so you don’t care if there are articles without a article_user_read relation — that can be optimized to a JOIN.

You also only want articles which have a comment after the user reads the article, so the relation between article, article_comment and comment must exist — that can be optimized to JOINs.

The main change however, is that you don’t need a correlated sub-query (referencing comment c in the sub-query), so the JOIN between article_user_read and article can be pushed to the main query.