UNION ALL MYSQL 真的很慢

Question

我在 mysql 中有一个查询，来自 2 select 个。当我单独运行这些查询时，它们运行在 1 秒内很快。但是，当我将它们与 union all 结合使用时，网站冻结并且在 union all 中执行相同的查询至少需要 20 秒。

知道为什么会这样吗？想不通。

查看下面的查询：

SELECT p.user_id, p.description, p.post_id, p.created_at, ps.username AS size_name, ps.username AS user_name, ps.avatar AS avatar, pz.title AS title, pz.slug AS slug 
FROM comments p 
JOIN users ps ON ps.id = p.user_id 
JOIN posts pz ON pz.id = p.post_id 

UNION ALL

SELECT p2.user_id, p2.description, p2.post_id, p2.created_at, ps2.username AS size_name, ps2.username AS user_name, ps2.avatar AS avatar, pz2.title AS title, pz2.slug AS slug 
FROM reply p2 
JOIN users ps2 ON ps2.id = p2.user_id 
JOIN posts pz2 ON pz2.id = p2.post_id 

order by created_at DESC 
LIMIT 10

Answer 1

UNION 默认为 UNION DISTINCT。如果你想要 UNION ALL，明确地说（你应该尽可能，因为 UNION DISTINCT 必须做很多工作才能使它们不同）。

另一件事是，order by 和 limit 作为一个整体应用于查询。您可能想让每个联合查询也有它们。见 https://dev.mysql.com/doc/refman/8.0/en/union.html:

ORDER BY and LIMIT in Unions

To apply an ORDER BY or LIMIT clause to an individual SELECT, parenthesize the SELECT and place the clause inside the parentheses:

(SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
UNION
(SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10);

如果您的快速个人版本确实有这些，那应该会有所帮助。所以像这样：

(SELECT p.user_id, p.description, p.post_id, p.created_at, ps.username AS size_name, ps.username AS user_name, ps.avatar AS avatar, pz.title AS title, pz.slug AS slug 
FROM comments p 
JOIN users ps ON ps.id = p.user_id 
JOIN posts pz ON pz.id = p.post_id 
order by created_at DESC 
LIMIT 10)

UNION ALL

(SELECT p2.user_id, p2.description, p2.post_id, p2.created_at, ps2.username AS size_name, ps2.username AS user_name, ps2.avatar AS avatar, pz2.title AS title, pz2.slug AS slug 
FROM reply p2 
JOIN users ps2 ON ps2.id = p2.user_id 
JOIN posts pz2 ON pz2.id = p2.post_id 
order by created_at DESC 
LIMIT 10)

order by created_at DESC 
LIMIT 10

Answer 2

在与UNION合并之前，减小每个子查询返回的表的大小。由于只想要前10个，所以每个子查询只需要前10个即可。

SELECT *
FROM (
    (SELECT p.user_id, p.description, p.post_id, p.created_at, ps.username AS size_name, ps.username AS user_name, ps.avatar AS avatar, pz.title AS title, pz.slug AS slug 
    FROM comments p 
    JOIN users ps ON ps.id = p.user_id 
    JOIN posts pz ON pz.id = p.post_id 
    ORDER BY created_at DESC
    LIMIT 10) 
    
    UNION ALL
    
    (
    SELECT p2.user_id, p2.description, p2.post_id, p2.created_at, ps2.username AS size_name, ps2.username AS user_name, ps2.avatar AS avatar, pz2.title AS title, pz2.slug AS slug 
    FROM reply p2 
    JOIN users ps2 ON ps2.id = p2.user_id 
    JOIN posts pz2 ON pz2.id = p2.post_id 
    ORDER BY created_at DESC
    LIMIT 10)
) AS x
order by created_at DESC 
LIMIT 10

UNION ALL MYSQL 真的很慢

UNION ALL MYSQL is really slow

mysql

sql

select

join

union-all