mysql explain时，possible_keys有索引，但是key为null没有使用索引
场景： 有三个表，表２连接表３查询很快，几毫秒；但是表２连接查询表４很慢，耗时几分钟。 注：都是用uid字段做关联
查询结果如下截图：
mysql> select concat_ws( '|' , '2016-08-23' , t3.country , count(distinct t2.id ) )
-> from t_reconnect_task t2 join incent.t_user_info t3
-> on ( t2.uid = t3.uid and date_format(t2.create_time,'%Y-%m-%d')= '2016-08-23' )
-> group by t3.country ;
+------------------------------------------------------------------------+
| concat_ws( '|' , '2016-08-23' , t3.country , count(distinct t2.id ) ) |
+------------------------------------------------------------------------+
| 2016-08-23|AR|6 |
| 2016-08-23|AT|5 |
| 2016-08-23|AU|2 |
| 2016-08-23|BD|6 |
| 2016-08-23|BG|3 |
| 2016-08-23|BR|36 |
| 2016-08-23|NI|2 |
| 2016-08-23|NL|7 |
| 2016-08-23|NZ|3 |
| 2016-08-23|US|1253 |
| 2016-08-23|UY|4 |
| 2016-08-23|VE|6 |
| 2016-08-23|ZA|3 |
+------------------------------------------------------------------------+
60 rows in set (0.09 sec)mysql> select concat_ws( '|' , '2016-08-23' , t4.ab_test , count(distinct t2.id ) )
-> from t_reconnect_task t2 join test_uid_abtest_ref t4
-> on ( t2.uid = t4.uid )
-> where date_format(t2.create_time,'%Y-%m-%d')= '2016-08-23'
-> group by t4.ab_test ;
+-------------------------------------------------------------------------+
| concat_ws( '|' , '2016-08-23' , t4.ab_test , count(distinct t2.id ) ) |
+-------------------------------------------------------------------------+
| 2016-08-23|-1|446 |
| 2016-08-23|0|1030 |
| 2016-08-23|1|939 |
+-------------------------------------------------------------------------+
3 rows in set (3 min 34.17 sec)ｅｘｐｌａｉｎ如下截图：
mysql>
mysql> explain
-> select concat_ws( '|' , '2016-08-23' , t4.ab_test , count(distinct t2.id ) )
-> from t_reconnect_task t2 join test_uid_abtest_ref t4
-> on ( t2.uid = t4.uid and date_format(t2.create_time,'%Y-%m-%d')= '2016-08-23' )
-> group by t4.ab_test ;
+----+-------------+-------+------+-------------------------+------+---------+------+-------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------------+------+---------+------+-------+---------------------------------+
| 1 | SIMPLE | t4 | ALL | test_uid_abtest_ref_ix1 | NULL | NULL | NULL | 16415 | Using temporary; Using filesort |
| 1 | SIMPLE | t2 | ALL | NULL | NULL | NULL | NULL | 25298 | Using where; Using join buffer |
+----+-------------+-------+------+-------------------------+------+---------+------+-------+---------------------------------+
2 rows in set (0.00 sec)
mysql> explain
-> select concat_ws( '|' , '2016-08-23' , t3.country , count(distinct t2.id ) )
-> from t_reconnect_task t2 join incent.t_user_info t3
-> on ( t2.uid = t3.uid and date_format(t2.create_time,'%Y-%m-%d')= '2016-08-23' )
-> group by t3.country ;
+----+-------------+-------+--------+---------------+------+---------+-----------------+-------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+------+---------+-----------------+-------+----------------------------------------------+
| 1 | SIMPLE | t2 | ALL | NULL | NULL | NULL | NULL | 25298 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | t3 | eq_ref | UID | UID | 194 | giftcard.t2.uid | 1 | Using where |
+----+-------------+-------+--------+---------------+------+---------+-----------------+-------+----------------------------------------------+
2 rows in set (0.00 sec)
分析： 其实possible_keys已经查询到了唯一键uid，但是key没有使用。对比表3和表4的区别：
CREATE TABLE `t_user_info` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`uid` varchar(64) COLLATE utf8_bin NOT NULL COMMENT '设备id',
`country` varchar(32) COLLATE utf8_bin NOT NULL COMMENT '国家',
`time_zone` int(2) NOT NULL COMMENT '时区',
`update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
UNIQUE KEY `UID` (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=17134 DEFAULT CHARSET=utf8 COLLATE=utf8_bin CREATE TABLE `test_uid_abtest_ref` (
`uid` varchar(64) NOT NULL COMMENT '用户id',
`ab_test` int(4) NOT NULL DEFAULT '0' COMMENT 'ab_test类型',
UNIQUE KEY `test_uid_abtest_ref_ix1` (`uid`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COMMENT='用户abtest类型临时表'
解决方法：
alter table test_uid_abtest_ref change uid `uid` varchar(64) COLLATE utf8_bin NOT NULL COMMENT '用户UID';验证：
mysql> select concat_ws( '|' , '2016-08-23' , t4.ab_test , count(distinct t2.id ) )
-> from t_reconnect_task t2 join test_uid_abtest_ref t4
-> on ( t2.uid = t4.uid and date_format(t2.create_time,'%Y-%m-%d')= '2016-08-23' )
-> group by t4.ab_test ;
+-------------------------------------------------------------------------+
| concat_ws( '|' , '2016-08-23' , t4.ab_test , count(distinct t2.id ) ) |
+-------------------------------------------------------------------------+
| 2016-08-23|-1|446 |
| 2016-08-23|0|1030 |
| 2016-08-23|1|939 |
+-------------------------------------------------------------------------+
3 rows in set (0.14 sec)