MySQL in查询优化

时间：2021-02-16 23:27:48

https://blog.****.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。

  1. 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');
  2. ......
  3. 30min+

然后我查询内部SQL,只需要3s+

  1. mysql> select distinct cdbh from sjkk_gcjl where jgsj>'2015-01-02 08:00:00' and jgsj<'2015-01-02 12:00:00';
  2. .....
  3. 1755 rows in set (3.30 sec)
  4. mysql> select count(*) from t_channel;
  5. ....
  6. 12062 rows in set (0.70 sec)

开发写的SQL为啥那么慢呢？看看执行计划

  1. 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');
  2. +----+--------------------+-----------+-------+---------------+----------+---------+------+--------+----------+------------------------------+
  3. | id | select_type        | table     | type  | possible_keys | key      | key_len | ref  | rows   | filtered | Extra                        |
  4. +----+--------------------+-----------+-------+---------------+----------+---------+------+--------+----------+------------------------------+
  5. |  1 | PRIMARY            | t_channel | ALL   | NULL          | NULL     | NULL    | NULL |  12062 |   100.00 | Using where                  |
  6. |  2 | DEPENDENT SUBQUERY | sjkk_gcjl | range | idx_jgsj      | idx_jgsj | 8       | NULL | 731868 |   100.00 | Using where; Using temporary |
  7. +----+--------------------+-----------+-------+---------------+----------+---------+------+--------+----------+------------------------------

看看数据库转换后的语句

  1. mysql> show warnings;
  2. SELECT
  3. `shanghai_full`.`t_channel`.`ID_` AS `ID_`,
  4. `shanghai_full`.`t_channel`.`Code_` AS `Code_`,
  5. ...... --这里会列出所有字段
  6. FROM
  7. `shanghai_full`.`t_channel`
  8. WHERE
  9. < in_optimizer > (
  10. `shanghai_full`.`t_channel`.`ID_` ,< EXISTS > (
  11. SELECT DISTINCT
  12. 1
  13. FROM
  14. `shanghai_full`.`sjkk_gcjl`
  15. WHERE
  16. (
  17. (
  18. `shanghai_full`.`sjkk_gcjl`.`jgsj` > '2015-01-02 08:00:00'
  19. )
  20. AND (
  21. `shanghai_full`.`sjkk_gcjl`.`jgsj` < '2015-01-02 12:00:00'
  22. )
  23. AND (
  24. < CACHE > (
  25. `shanghai_full`.`t_channel`.`ID_`
  26. ) = `shanghai_full`.`sjkk_gcjl`.`cdbh`
  27. )
  28. )
  29. )
  30. );

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）

  1. 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;
  2. ......
  3. 1264 rows in set (3.30 sec)
  4. 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;
  5. +----+-------------+------------+--------+---------------+----------+---------+---------+--------+----------+------------------------------+
  6. | id | select_type | table      | type   | possible_keys | key      | key_len | ref     | rows   | filtered | Extra                        |
  7. +----+-------------+------------+--------+---------------+----------+---------+---------+--------+----------+------------------------------+
  8. |  1 | PRIMARY     | <derived2> | ALL    | NULL          | NULL     | NULL    | NULL    |   1755 |   100.00 |                              |
  9. |  1 | PRIMARY     | t1    www.boshenyl.cn      | eq_ref | PRIMARY,ID_   | PRIMARY  | 74      | t2.cdbh |      1 |   100.00 | Using where                  |
  10. |  2 | DERIVED     | sjkk_gcjl  | range  | idx_jgsj      | idx_jgsj | 8       | NULL    | 731868 |   100.00 | Using where; Using temporary |
  11. +----+-------------+------------+--------+---------------+----------+---------+---------+--------+----------+------------------------------+
  12. mysql> show warnings;
  13. SELECT
  14. `shanghai_full`.`t1`.`ID_` AS `ID_`,
  15. ........ ---这里会列出所有字段FROM
  16. `shanghai_full`.`t_channel` `t1`
  17. JOIN (
  18. SELECT DISTINCT
  19. `shanghai_full`.`sjkk_gcjl`.`cdbh` AS `www.cnzhaotai.com cdbh`
  20. FROM
  21. `shanghai_full`.www.fengshen157.com `sjkk_gcjl`
  22. WHERE
  23. (
  24. (
  25. `shanghai_full`.`sjkk_gcjl`.`jgsj` > '2015-01-02 08:00:00'
  26. )
  27. AND (
  28. `shanghai_full`.`sjkk_gcjl`.`jgsj` < '2015-01-02 12:00:00'
  29. )
  30. )
  31. ) `t2`
  32. WHERE
  33. (
  34. `shanghai_full`.`t1`.`ID_` = www.taohuayuan178.com `t2`.`cdbh`
  35. );

优化方式二（使用memory引擎的临时表）

  1. 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');
  2. Query OK, 1755 rows affected (9.00 sec)
  3. Records: 1755  Duplicates: 0  Warnings: 0
  4. mysql> select * from t_channel where id_ in(select * from tmp_channel);
  5. .....
  6. 1264 rows in set (0.26 sec)
  7. mysql> explain extended select * from t_channel where id_ in(select * from tmp_channel);
  8. +----+--------------------+-------------+------+---------------+------+---------+------+------+----------+-------------+
  9. | id | select_type        | table       | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
  10. +----+--------------------+-------------+------+---------------+------+---------+------+------+----------+-------------+
  11. |  1 | PRIMARY            | t_channel   | ALL  | NULL          | NULL | NULL    | NULL | 3224 |   100.00 | Using where |
  12. |  2 | DEPENDENT SUBQUERY | tmp_channel | ALL  | NULL          | NULL | NULL    | NULL |   20 |   100.00 | Using where |
  13. +----+--------------------+-------------+------+---------------+------+---------+------+------+----------+-------------+
  14. mysql> show warnings;
  15. | 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，

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