如何解决UNION ALL MYSQL 真的很慢
我在 MysqL 中有一个查询,有 2 个选择。当我单独运行这些查询时,它们会在 1 秒内快速运行。但是当我将它们与 union all 结合起来时,网站会冻结,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
解决方法
在与UNION
合并之前,减少每个子查询返回的表的大小。由于您只想要前 10 个,因此您只需要每个子查询的前 10 个。
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 默认为 UNION DISTINCT。如果您想要 UNION ALL,请明确说明(并且您应该尽可能,因为 UNION DISTINCT 必须做很多工作才能使它们与众不同)。
另一件事是 order by 和 limit 适用于整个查询。您可能想让每个联合查询也包含它们。见https://dev.mysql.com/doc/refman/8.0/en/union.html:
联合中的 ORDER BY 和 LIMIT
要将 ORDER BY 或 LIMIT 子句应用于单个 SELECT,请将 SELECT 括起来并将子句放在括号内:
(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。所以是这样的:
(SELECT p.user_id,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,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
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