什么是復合索引，如何正確使用？

Question

我有一個非常慢的查詢，重復了很多次。 我嘗試索引各個字段，但似乎無濟於事。 CPU使用率仍然很高，查詢仍然顯示在慢查詢日志中。 似乎我需要復合索引？

如何正確索引以下查詢？

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;

解釋結果： 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 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

您想要在to_attachments(object_type, object_id, attachment_id)和attachments(id)上to_attachments(object_type, object_id, attachment_id)索引。

Answer 2

您的索引順序錯誤，應該是（object_type，object_id，attachment_id）。 在多列索引中，索引中列的順序為MATTER。