GROUP_CONCAT太慢

Question

你好，我添加了GROUP_CONCAT函数到我的查询和该函数杀死了我的查询：/。 我的查询是：

SELECT u.username,a.user_id,a.id,a.text,a.lang as fromLang,b.lang as toLang,GROUP_CONCAT(DISTINCT b.id) AS translation_ids FROM sentence as a 
    INNER JOIN sentence_relationship as sr ON 
    (sr.sentence_id = a.id) 
    INNER JOIN sentence as b ON 
    (b.id = sr.translation_id AND a.id = sr.sentence_id) 
    INNER JOIN users as u ON 
    (u.id = a.user_id) GROUP BY a.id LIMIT 10; 

该查询有什么问题？

Answer 1

这是您的查询（有点格式化）：

SELECT u.username, a.user_id, a.id,a.text,a.lang as fromLang, b.lang as toLang, 
     GROUP_CONCAT(DISTINCT b.id) AS translation_ids 
FROM sentence a INNER JOIN 
    sentence_relationship sr 
    ON sr.sentence_id = a.id INNER JOIN 
    sentence b 
    ON b.id = sr.translation_id AND a.id = sr.sentence_id INNER JOIN 
    users as u 
    ON u.id = a.user_id 
GROUP BY a.id 
LIMIT 10; 

它是从你的问题的group_concat()是否与group by加入不清楚。这可能会减慢速度。

limit 10正在匹配的第一个10 a.id（group by做了一个隐式排序）。如果使用子查询执行此操作，它可能会加快查询速度：

SELECT u.username, a.user_id, a.id,a.text,a.lang as fromLang, b.lang as toLang, 
     GROUP_CONCAT(DISTINCT b.id) AS translation_ids 
FROM (select s.* 
     from sentence s 
     order by a.id 
     limit 10 
    ) a INNER JOIN 
    sentence_relationship sr 
    ON sr.sentence_id = a.id INNER JOIN 
    sentence b 
    ON b.id = sr.translation_id AND a.id = sr.sentence_id INNER JOIN 
    users as u 
    ON u.id = a.user_id 
GROUP BY a.id; 

这假设所有连接都起作用并匹配记录。如果连接用于过滤，则可能会返回少于10行。