获取消息的此查询的最佳索引

Question

我有以下查询获取每个用户收到的最新消息（＆＃34;收件箱＆＃34;查询）：

select SQL_NO_CACHE msg.*, msg.id as msg_id, t2.count_msgs, usr.gender,  usr.id,
            msg.senderid as refuid from mailbox msg
inner join (select senderid,max(id) as last_msg_id,count(id) as count_msgs from mailbox where recipientid='893720' and owner='893720' and folder='inbox' group by senderid) t2 on 
t2.senderid = msg.senderid and t2.last_msg_id = msg.id
left join user usr on usr.id = msg.senderid
 where recipientid = '893720' order by msg.sendtime desc;

我目前有一个邮箱索引（所有者，文件夹，senderid）。解释输出：

非常感谢任何帮助。

Answer 1

这是您的查询：

select SQL_NO_CACHE msg.*, msg.id as msg_id, t2.count_msgs, usr.gender,  usr.id,
        msg.senderid as refuid
from mailbox msg inner join
     (select senderid, max(id) as last_msg_id, count(id) as count_msgs 
      from mailbox
      where recipientid = '893720' and owner = '893720' and folder = 'inbox'
      group by senderid
     ) t2
     on t2.senderid = msg.senderid and t2.last_msg_id = msg.id left join
     user usr
     on usr.id = msg.senderid
where msg.recipientid = '893720'
order by msg.sendtime desc;

此版本查询的最佳索引是：mailbox(recipientid, owner, folder, senderid, id)，mailbox（senderid，id）, and user（id）`。

第一个索引是子查询的覆盖索引（意味着它包含列）。密钥按where子句排序。 join需要第二个和第三个。第三个可能已经存在。

请注意，如果recipientid和owner确实是数字，则不要对值使用单引号。有时，使用错误的类型可能会混淆SQL优化器。