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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MySQL> show create table test06 \G
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*************************** 1. row ***************************
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Table: test06
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Create Table: CREATE TABLE `test06` (
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`id` bigint(11) NOT NULL DEFAULT '0',
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`u_id` bigint(11) NOT NULL,
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`openid` varchar(100) DEFAULT NULL,
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`unionid` varchar(100) DEFAULT NULL,
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`username` varchar(100) NOT NULL,
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`password` varchar(100) NOT NULL,
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`create_time` datetime NOT NULL,
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KEY `idx_test03_id` (`id`),
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KEY `idx_test03_name` (`username`),
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KEY `idx_test06_crea_time` (`create_time`)
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) ENGINE=InnoDB DEFAULT CHARSET=utf8
使用函数进行查询：
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mysql> select count(*) from test06 where date(create_time)=curdate();
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+----------+
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| count(*) |
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+----------+
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| 0 |
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+----------+
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1 row in set (1.00 sec)
不使用函数：
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mysql> select count(*) from test06 where create_time=date_format(curdate(),'%Y-%m-%d');
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+----------+
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| count(*) |
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+----------+
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| 0 |
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+----------+
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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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mysql> explain select count(*) from test06 where date(create_time)=curdate();
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+----+-------------+--------+-------+---------------+----------------------+---------+------+---------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+--------+-------+---------------+----------------------+---------+------+---------+--------------------------+
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| 1 | SIMPLE | test06 | index | NULL | idx_test06_crea_time | 5 | NULL | 2009559 | Using where; Using index |
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+----+-------------+--------+-------+---------------+----------------------+---------+------+---------+--------------------------+
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1 row in set (0.00 sec)
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mysql> explain select count(*) from test06 where create_time=date_format(curdate(),'%Y-%m-%d');
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+----+-------------+--------+------+----------------------+----------------------+---------+-------+------+-------------+
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| id | select_type | table | type | possible_keys key | key_len | ref | rows | Extra |
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+----+-------------+--------+------+----------------------+----------------------+---------+-------+------+-------------+
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| 1 | SIMP | test06 | ref | idx_test06_crea_time | idx_test06_crea_time | 5 | const | 1 | Using index |
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+----+-------------+--------+------+----------------------+----------------------+---------+-------+------+-------------+
当前名称：字段使用函数索引被抑制
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