字段使用函数索引被抑制

 在索引字段上使用函数，该字段的索引将会被抑制。如下案例：
查看表结构：

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1. mysql> show create table test06 \G
   
2. *************************** 1. row ***************************
3.        Table: test06
4. Create Table: CREATE TABLE `test06` (
5.   `id` bigint(11) NOT NULL DEFAULT '0',
6.   `u_id` bigint(11) NOT NULL,
7.   `openid` varchar(100) DEFAULT NULL,
8.   `unionid` varchar(100) DEFAULT NULL,
9.   `username` varchar(100) NOT NULL,
10.   `password` varchar(100) NOT NULL,
11.   `create_time` datetime NOT NULL,
12.   KEY `idx_test03_id` (`id`),
13.   KEY `idx_test03_name` (`username`),
14.   KEY `idx_test06_crea_time` (`create_time`)
15. ) ENGINE=InnoDB DEFAULT CHARSET=utf8

使用函数进行查询：

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1. mysql> select count(*) from test06 where date(create_time)=curdate();
   
2. +----------+
3. | count(*) |
4. +----------+
5. | 0 |
6. +----------+
7. 1 row in set (1.00 sec)

不使用函数：

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1. mysql> select count(*) from test06 where create_time=date_format(curdate(),'%Y-%m-%d');
   
2. +----------+
3. | count(*) |
4. +----------+
5. | 0 |
6. +----------+
7. 1 row in set (0.03 sec)

  可以看出：查询时间变快很多。
 对比一下执行计划：

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1. mysql> explain select count(*) from test06 where date(create_time)=curdate();
   
2. +----+-------------+--------+-------+---------------+----------------------+---------+------+---------+--------------------------+
3. | id | select_type | table  | type  | possible_keys | key                  | key_len | ref  | rows    | Extra                    |
4. +----+-------------+--------+-------+---------------+----------------------+---------+------+---------+--------------------------+
5. | 1  | SIMPLE      | test06 | index | NULL          | idx_test06_crea_time | 5       | NULL | 2009559 | Using where; Using index |
6. +----+-------------+--------+-------+---------------+----------------------+---------+------+---------+--------------------------+
7. 1 row in set (0.00 sec)
9. mysql> explain select count(*) from test06 where create_time=date_format(curdate(),'%Y-%m-%d');
10. +----+-------------+--------+------+----------------------+----------------------+---------+-------+------+-------------+
11. | id | select_type | table  | type | possible_keys    key | key_len              | ref     | rows  | Extra              |
12. +----+-------------+--------+------+----------------------+----------------------+---------+-------+------+-------------+
13. | 1  | SIMP        | test06 | ref  | idx_test06_crea_time | idx_test06_crea_time | 5       | const | 1    | Using index |
14. +----+-------------+--------+------+----------------------+----------------------+---------+-------+------+-------------+