mysql - 什么是复合索引以及如何正确使用它？

Question

我有一个非常慢的查询，它会重复很多次。我试过索引各个字段，但似乎没有帮助。CPU使用率仍然很高，查询仍然出现在慢查询日志中。看来我需要一个复合索引？

如何正确索引以下查询？

select *
from `to_attachments` left join
     `attachments`
     on `to_attachments`.`attachment_id` = `attachments`.`id`
where `to_attachments`.`object_type` = 'communicator' and `to_attachments`.`object_id` = '64328'
order by `attachments`.`created_at` desc;

解释结果： 1 SIMPLE to_attachments index NULL PRIMARY 775 NULL 244384 Using where; Using index; Using temporary; Using filesort 1 SIMPLE attachments eq_ref PRIMARY PRIMARY 4 quote.to_attachments.attachment_id 1 NULL

to_attachments 的索引

Answer 1

你想要索引to_attachments(object_type, object_id, attachment_id)和attachments(id)。

Answer 2

您的索引顺序错误应该是（object_type，object_id，attachment_id）。在多列索引中，索引中列的顺序是 MATTER。