记自己一次sql优化（使用伪列）
起因
未优化之前的s q l,大表内连接次数太多，严重影响效率。因此需要优化。
sql 要满足的逻辑
以status_id分组，weight_id最大的行中（同一组中可能有好几个weight_id相同的行），mid最大的那一行
表结构
CREATE TABLE `at_x_0` (
`uixd` ,
`mixd` ,
`status_ixd` ,
`message_type` ,
`time_ixd` ,
`time` ,
`weight_id` ,
`extension` ,
UNIQUE KEY `uid` (`mid`,`uid`),
KEY `uid_2` (`uid`,`time_id`,`message_type`,`status_id`,`weight_id`),
KEY `sw` (`status_id`,`weight_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8优化前的表结构（两次内连接）
“select e.* from db . table e inner join
(
select max(c.mid) mid from
(select a.mid,a.status_id,a.weight_id
from db . table a inner join
(select f.status_id,max(f.weight_id) weight_id
from at_message_0.at_message_0 f where f.uid=? and f.time_id=? and f.message_type=? and f.weight_id<=? group by f.status_id
) b on a.status_id=b.status_id and a.uid=? and a.message_type=? and a.weight_id=b.weight_id order by a.weight_id desc
) c group by c.status_id
) d where e.mid=d.mid and e.uid=? order by e.weight_id desc limit ?,?”;
优化前的执行计划
+------+-------------+------------+-------+---------------+-------+---------+------+-------+-------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+------------+-------+---------------+-------+---------+------+-------+-------------------------------------------------+
| 1 | SIMPLE | | ALL | NULL | NULL | NULL | NULL | 50176 | Using temporary; Using filesort |
| 2 | DERIVED | | ALL | NULL | NULL | NULL | NULL | 224 | Using temporary; Using filesort |
| 2 | DERIVED | b | ALL | NULL | NULL | NULL | NULL | 224 | Using where; Using join buffer (flat, BNL join) |
| 4 | DERIVED | a | index | NULL | uid_2 | 29 | NULL | 224 | Using index; Using temporary; Using filesort |
+------+-------------+------------+-------+---------------+-------+---------+------+-------+-------------------------------------------------+
优化后的sql(一次右连接，加伪列)
set @num :=0, @group := ”;
select x.uid,x.mid,x.status_id,x.message_type,x.time_id,x.time,x.weight_id,x.extension from
(
select d.*,
@num := if(@group = d.status_id, @num:=@num-1, 0) as row_number,
@group := d.status_id as dummy from
(select b.* from at_message_0.at_message_0 b
right join
(select a.status_id,max(a.weight_id) as max_weight_id from at_message_0.at_message_0 a group by a.status_id) c
on b.status_id = c.status_id and b.weight_id= c.max_weight_id
) d order by d.status_id,d.mid desc
) as x group by x.status_id order by x.row_number desc;
优化后的执行计划
+------+-------------+------------+-------+---------------+------+---------+-----------------------------+------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+------------+-------+---------------+------+---------+-----------------------------+------+---------------------------------+
| 1 | SIMPLE | | ALL | NULL | NULL | NULL | NULL | 201 | Using temporary; Using filesort |
| 2 | DERIVED | | ALL | NULL | NULL | NULL | NULL | 201 | Using temporary; Using filesort |
| 2 | DERIVED | b | ref | sw,sm | sw | 16 | c.status_id,c.max_weight_id | 1 | Using where |
| 4 | DERIVED | a | range | NULL | sw | 8 | NULL | 202 | Using index for group-by |
+------+-------------+------------+-------+---------------+------+---------+-----------------------------+------+---------------------------------+