mysql select with in子句不使用索引 [英] Mysql select with in clause does not use index

问题描述

我有一个联系人表，其主键为id。它还有一个辅助索引idx_id_del_user（id，deleted，user_id）。

I have a contacts table which has a primary key of id. It also has a secondary index idx_id_del_user (id, deleted, user_id).

以下查询使用索引，因此非常快 -

The following query uses the index and therefore is very fast -

select id
from jts_contacts
where id = '00000402-25c8-7375-e3df-4ec5b66de11d'
and deleted = 0;

在0.0098s中获取1行

1 row fetched in 0.0098s

但是，当我使用in子句时，外部查询进入全表扫描。我希望它使用主键或idx_id_del_user。

However when I use the in clause the outer query goes into a full table scan. I am expecting it to use either the primary key or the idx_id_del_user.

select  *
from jts_contacts FORCE INDEX (idx_id_del_user)
where id in
(select id
from jts_contacts
where id = '00000402-25c8-7375-e3df-4ec5b66de11d')
and deleted = 0

在9s内获取1行

解释计划 -

id, select_type,          table,         type, possible_keys,                 key, key_len, ref, rows, Extra
------------------------------------------------------------------------------------
1, 'PRIMARY',            'jts_contacts', 'ALL', '',                           '',   '',     '', 1127275, 'Using where'
2, 'DEPENDENT SUBQUERY', 'jts_contacts', 'const', 'PRIMARY,idx_id_del_user', 'PRIMARY', '108', 'const', 1, 'Using index'

此表有120万条记录和t他对桌子进行了分析。我没有使用FORCE INDEX选项尝试过它，但它仍然没有使用索引。有关更快地进行此查询的建议吗？

This table has 1.2 million records and the table was analyzed. I have tried it without the FORCE INDEX option, but it is still not using the index. Any suggestions on making this query faster?

警告：使用连接代替in子句将起作用，但是因为这是来自现有产品的生成查询 - 它无法更改为使用连接。

Caveat: using a join instead of the in clause will work, however since this is a generated query from a existing product - it cannot be changed to use joins.

推荐答案

据我所知， IN 遍历所有匹配的记录，并将它们与子句中的值进行比较，一次一行。

From what I can tell, IN goes through all of the matching records and compares them to the values in the clause, one row at a time.

所以，你能做的最好的是使用已删除上的索引，您只会查看 deleted = 0 的记录。

So, the best you can do is use an index on deleted and you'll only be going through the records where deleted = 0.

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