php - GROUP_CONCAT 太慢

Question

您好，我在查询中添加了 GROUP_CONCAT 函数，该函数终止了我的查询:/。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(有些格式):

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;

从您的问题中不清楚group_concat()是否与group by一起添加。这可能会减慢速度。

limit 10 取前 10 个匹配的 a.id(group by 执行隐式排序)。如果您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;

这假设所有连接都有效并且匹配记录。如果连接用于过滤，那么您返回的数据可能会少于 10 行。