字段使用函数索引被抑制

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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

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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)

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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)

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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)
   
8. 
   
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. +----+-------------+--------+------+----------------------+----------------------+---------+-------+------+-------------+