https://blog.csdn.net/gua___gua/article/details/47401621

MySQL in查询优化<一>

原创 2015年08月10日 17:57:56

5137

开发说他写了个SQL特别慢，让看看。

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select * from t_channel where id_ in(select distinct cdbh from sjkk_gcjl where jgsj>'2015-01-02 08:00:00' and jgsj<'2015-01-02 12:00:00');
......
30min+

然后我查询内部SQL,只需要3s+

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mysql> select distinct cdbh from sjkk_gcjl where jgsj>'2015-01-02 08:00:00' and jgsj<'2015-01-02 12:00:00';
.....
1755 rows in set (3.30 sec)
mysql> select count(*) from t_channel;
....
12062 rows in set (0.70 sec)

开发写的SQL为啥那么慢呢？看看执行计划

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explain extended select * from t_channel where id_ in(select distinct cdbh from sjkk_gcjl where jgsj>'2015-01-02 08:00:00' and jgsj<'2015-01-02 12:00:00');
+----+--------------------+-----------+-------+---------------+----------+---------+------+--------+----------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-----------+-------+---------------+----------+---------+------+--------+----------+------------------------------+
| 1 | PRIMARY | t_channel | ALL | NULL | NULL | NULL | NULL | 12062 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | sjkk_gcjl | range | idx_jgsj | idx_jgsj | 8 | NULL | 731868 | 100.00 | Using where; Using temporary |
+----+--------------------+-----------+-------+---------------+----------+---------+------+--------+----------+------------------------------

看看数据库转换后的语句

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mysql> show warnings;
SELECT
`shanghai_full`.`t_channel`.`ID_` AS `ID_`,
`shanghai_full`.`t_channel`.`Code_` AS `Code_`,
...... --这里会列出所有字段
FROM
`shanghai_full`.`t_channel`
WHERE
< in_optimizer > (
`shanghai_full`.`t_channel`.`ID_` ,< EXISTS > (
SELECT DISTINCT
1
FROM
`shanghai_full`.`sjkk_gcjl`
WHERE
(
(
`shanghai_full`.`sjkk_gcjl`.`jgsj` > '2015-01-02 08:00:00'
)
AND (
`shanghai_full`.`sjkk_gcjl`.`jgsj` < '2015-01-02 12:00:00'
)
AND (
< CACHE > (
`shanghai_full`.`t_channel`.`ID_`
) = `shanghai_full`.`sjkk_gcjl`.`cdbh`
)
)
)
);

可见，经过mysql优化器后，in 给转换成exists的方式（mysql认为换成exists更快，呵呵）。

慢的原因：走了exists，查询把外表t_channel做为主表进行全表扫描，每扫描一行数据，然后对子查询进行查询看这条数据是否符合条件。

特别说明：mysql版本是5.5。在5.6中mysql做了改进，将in的这种查询转换为了join。

优化方式一（将in查询转换为连接查询）

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select t1.* from t_channel t1,(select distinct cdbh from sjkk_gcjl where jgsj>'2015-01-02 08:00:00' and jgsj<'2015-01-02 12:00:00') t2 where t1.id_=t2.cdbh;
......
1264 rows in set (3.30 sec)
mysql> explain extended select t1.* from t_channel t1,(select distinct cdbh from sjkk_gcjl where jgsj>'2015-01-02 08:00:00' and jgsj<'2015-01-02 12:00:00') t2 where t1.id_www.255055.cn/ =t2.cdbh;
+----+-------------+------------+--------+---------------+----------+---------+---------+--------+----------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+--------+---------------+----------+---------+---------+--------+----------+------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 1755 | 100.00 | |
| 1 | PRIMARY | t1 www.boshenyl.cn | eq_ref | PRIMARY,ID_ | PRIMARY | 74 | t2.cdbh | 1 | 100.00 | Using where |
| 2 | DERIVED | sjkk_gcjl | range | idx_jgsj | idx_jgsj | 8 | NULL | 731868 | 100.00 | Using where; Using temporary |
+----+-------------+------------+--------+---------------+----------+---------+---------+--------+----------+------------------------------+
mysql> show warnings;
SELECT
`shanghai_full`.`t1`.`ID_` AS `ID_`,
........ ---这里会列出所有字段FROM
`shanghai_full`.`t_channel` `t1`
JOIN (
SELECT DISTINCT
`shanghai_full`.`sjkk_gcjl`.`cdbh` AS `www.cnzhaotai.com cdbh`
FROM
`shanghai_full`.www.fengshen157.com `sjkk_gcjl`
WHERE
(
(
`shanghai_full`.`sjkk_gcjl`.`jgsj` > '2015-01-02 08:00:00'
)
AND (
`shanghai_full`.`sjkk_gcjl`.`jgsj` < '2015-01-02 12:00:00'
)
)
) `t2`
WHERE
(
`shanghai_full`.`t1`.`ID_` = www.taohuayuan178.com `t2`.`cdbh`
);

优化方式二（使用memory引擎的临时表）

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mysql> create temporary table tmp_www.yibaoyule1.com channel engine=memory (select distinct cdbh from sjkk_gcjl where jgsj>'2015-01-02 08:00:00' and jgsj<'2015-01-02 12:00:00');
Query OK, 1755 rows affected (9.00 sec)
Records: 1755 Duplicates: 0 Warnings: 0
mysql> select * from t_channel where id_ in(select * from tmp_channel);
.....
1264 rows in set (0.26 sec)
mysql> explain extended select * from t_channel where id_ in(select * from tmp_channel);
+----+--------------------+-------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | t_channel | ALL | NULL | NULL | NULL | NULL | 3224 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | tmp_channel | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | Using where |
+----+--------------------+-------------+------+---------------+------+---------+------+------+----------+-------------+
mysql> show warnings;
| Note | 1003 | select `vmc_jiaqi`.`t_channel`.`ID_` AS `ID_`,`vmc_jiaqi`.`t_channel`.`Code_` AS `Code_`,`vmc_jiaqi`.`t_channel`.`HostId_` AS `HostId_`,`vmc_jiaqi`.`t_channel`.`RoadMonitorStationId_` AS `RoadMonitorStationId_`,`vmc_jiaqi`.`t_channel`.`ChannelNo_` AS `ChannelNo_`,`vmc_jiaqi`.`t_channel`.`Name_` AS `Name_`,`vmc_jiaqi`.`t_channel`.`ChannelType_` AS `ChannelType_`,`vmc_jiaqi`.`t_channel`.`Ext_` AS `Ext_`,`vmc_jiaqi`.`t_channel`.`DeviceAddress_` AS `DeviceAddress_`,`vmc_jiaqi`.`t_channel`.`MinSpeed_`www.mhylpt.comS `MinSpeed_`,`vmc_jiaqi`.`t_channel`.`MaxSpeed_` AS `MaxSpeed_`,`vmc_jiaqi`.`t_channel`.`LimitMinRatio_` AS `LimitMinRatio_`,`vmc_jiaqi`.`t_channel`.`LimitMaxRatio_` AS `LimitMaxRatio_`,`vmc_jiaqi`.`t_channel`.`Direction_` AS `Direction_`,`vmc_jiaqi`.`t_channel`.`JcDirection_` AS `JcDirection_`,`vmc_jiaqi`.`t_channel`.`LaneNum_` AS `LaneNum_`,`vmc_jiaqi`.`t_channel`.`FrameDropNum_` AS `FrameDropNum_`,`vmc_jiaqi`.`t_channel`.`RecogizeRectLeft_` AS `RecogizeRectLeft_`,`vmc_jiaqi`.`t_channel`.`RecogizeRectTop_` AS `RecogizeRectTop_`,`vmc_jiaqi`.`t_channel`.`RecogizeRectWidth_` AS `RecogizeRectWidth_`,`vmc_jiaqi`.`t_channel`.`RecogizeRectHeight_` AS `RecogizeRectHeight_`,`vmc_jiaqi`.`t_channel`.`RmpServerIp_` AS `RmpServerIp_`,`vmc_jiaqi`.`t_channel`.`RmpServerPort_` AS `RmpServerPort_`,`vmc_jiaqi`.`t_channel`.`VirtualGateServerId_` AS `VirtualGateServerId_`,`vmc_jiaqi`.`t_channel`.`LaneType_` AS `LaneType_`,`vmc_jiaqi`.`t_channel`.`DeviceType_` AS `DeviceType_`,`vmc_jiaqi`.`t_channel`.`ManufacturerId_` AS `ManufacturerId_`,`vmc_jiaqi`.`t_channel`.`IsLocalSavePicture_` AS `IsLocalSavePicture_`,`vmc_jiaqi`.`t_channel`.`OrderNO_` AS `OrderNO_`,`vmc_jiaqi`.`t_channel`.`channelStatus` AS `channelStatus` from `vmc_jiaqi`.`t_channel` where <in_optimizer>(`vmc_jiaqi`.`t_channel`.`ID_`,<exists>(select 1 from `vmc_jiaqi`.`tmp_channel` where (<cache>(`vmc_jiaqi`.`t_channel`.`ID_`) = `vmc_jiaqi`.`tmp_channel`.`cdbh`)))

注意：第二种方式还是使用了exists的执行方式，所以这种方式没有第一种方式好，在特定的条件下可能会有用处，

MySQL in查询优化

MySQL in查询优化<一>

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