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如何用索引优化查询?

innodb left-join select sql mysql

1

Wonka · 技术社区 · 8 年前

我们正在尝试从数据库中获取跟踪者的最新10个通知。我们做了一些连接来确保为跟踪者获得正确的通知集。如果他们跟踪的人(他们的领导)添加了一个新的帖子,那么跟随者应该只得到在他们开始跟踪领导之后添加的帖子的通知(没有意义将他们的领导的旧帖子显示为新的通知)。另一个加入是为了确保我们得到通知 read_at 时间,所以追随者知道它是否已经被阅读。这是查询,但需要 ~9秒 太慢了。理想情况下只需几毫秒,特别是索引:

查询:

SELECT nf.id, nf.uuid, nf.leader_id, nf.data, nf.created_at, nfr.read_at
FROM notification_followers nf
LEFT JOIN user_follows uf ON uf.leader_id = nf.leader_id AND uf.follower_id = 14 AND uf.follow_status = 'follow'
LEFT JOIN notification_followers_read nfr ON nf.id = nfr.notification_followers_id AND nfr.follower_id = 14
WHERE (nf.created_at > uf.created_at)
ORDER BY nf.id DESC
LIMIT 10

索引:

ALTER TABLE `notification_followers` ADD INDEX `nf_lid_ca_id_idx` (`leader_id`,`created_at`,`id`);
ALTER TABLE `user_follows` ADD KEY`uf_fid_lid_fs_ca_idx` (`follower_id`,`leader_id`,`follow_status`,`created_at`)
ALTER TABLE `notification_followers_read` ADD INDEX `nfr_fid_nfid_ra_idx` (`follower_id`,`notification_followers_id`,`read_at`);

解释:

正确的结果(大约9秒):

SQL转储:

SQL DUMP TO REPRODUCE LOCALLY 只需创建 speed_test 本地数据库和导入文件以查看所有表数据的慢速查询问题 (约10万行) .

我们如何在几毫秒内优化上述内容以获得正确的结果?

2 回复 | 直到 8 年前

1

3

Gordon Linoff 8 年前

对于此查询:

SELECT nf.id, nf.uuid, nf.leader_id, nf.data, nf.created_at, nfr.read_at
FROM notification_followers nf JOIN
     user_follows uf 
     ON uf.leader_id = nf.leader_id AND uf.follower_id = 14 AND
        uf.follow_status = 'follow' LEFT JOIN 
     notification_followers_read nfr
     ON nf.id = nfr.notification_followers_id AND nfr.follower_id = 14
WHERE nf.created_at > uf.created_at
ORDER BY nf.id DESC
LIMIT 10;

我建议索引 user_follower(leader_id, follower_id, follow_status, created_at) 和 notification_followers_read(notification_followers_id, follower_id, read_at) . 索引中列的顺序很重要。

注意我改变了第一个 JOIN 内部连接,因为 WHERE 无论如何从句都会把它变为一个。

嗯,让我们尝试重写查询:

SELECT nf.id, nf.uuid, nf.leader_id, nf.data, nf.created_at,
       (SELECT nfr.read_at
        FROM notification_followers_read nfr
        WHERE nf.id = nfr.notification_followers_id AND nfr.follower_id = 14
       ) nfr
FROM (SELECT nf.*
      FROM notification_followers nf 
      WHERE EXISTS (SELECT 1
                    FROM user_follows uf 
                    WHERE uf.leader_id = nf.leader_id AND uf.follower_id = 14 AND
                          uf.follow_status = 'follow' AND nf.created_at > uf.created_at
                   )
      ORDER BY nf.id DESC
      LIMIT 10
     ) nf;

为此,您需要确保在 notification_followers(id) 也。

根据您的数据,使用此方法内部子查询可能更快:

FROM (SELECT nf.*
      FROM user_follows uf JOIN
           notification_followers nf 
           ON uf.leader_id = nf.leader_id AND nf.created_at > uf.created_at
      WHERE uf.follower_id = 14 AND uf.follow_status = 'follow' 
      ORDER BY nf.id DESC
      LIMIT 10
     ) nf

为此,索引是 user_follows(follower_id, follow_status, leader_id, created_at) 和 notification_followers(leader_id, created_at, id) . 这可能更快。

2

0

rajat 8 年前

你应该试试这个。

SELECT nf.id, nf.uuid, nf.leader_id, nf.data, nf.created_at, nfr.read_at 
FROM notification_followers nf 
JOIN user_follows uf ON uf.leader_id = nf.leader_id and nf.created_at > uf.created_at AND uf.follow_status = 'follow'  AND uf.follower_id = 14 
LEFT JOIN notification_followers_read nfr ON nf.id = nfr.notification_followers_id AND nfr.follower_id = 14 
ORDER BY nf.id DESC
LIMIT 10;

在上创建索引。

ALTER TABLE `notification_followers` ADD INDEX `nf_lid_ca_id_idx`(`leader_id`,`created_at`,`id`);
ALTER TABLE `user_follows` ADD KEY`uf_fid_lid_fs_ca_idx`(`leader_id`,`created_at`,`follow_status`,`follower_id`)
ALTER TABLE `notification_followers_read` ADD INDEX `nfr_fid_nfid_ra_idx`(`notification_followers_id`,`follower_id`,`read_at`);