GROUP_CONCAT太慢

Question

你好，我添加了GROUP_CONCAT函數到我的查詢和該函數殺死了我的查詢：/。 我的查詢是：

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; 

該查詢有什麼問題？

Answer 1

這是您的查詢（有點格式化）：

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; 

它是從你的問題的group_concat()是否與group by加入不清楚。這可能會減慢速度。

limit 10正在匹配的第一個10 a.id（group by做了一個隱式排序）。如果使用子查詢執行此操作，它可能會加快查詢速度：

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; 

這假設所有連接都起作用並匹配記錄。如果連接用於過濾，則可能會返回少於10行。