use testdb; Database changed mysql> create table tb_function(id int primary key auto_increment,name varchar(100),create_time datetime); Query OK, 0 rows affected (0.01 sec) mysql> insert into tb_function(name,creatE_time) values(‘anniuadaOAIFAPUHIA‘,‘2020-07-01 12:00:00‘); Query OK, 1 row affected (0.02 sec) mysql> insert into tb_function(name,creatE_time) values(‘CWQSsar3qcssg‘,‘2020-07-01 15:00:00‘); Query OK, 1 row affected (0.01 sec) mysql> insert into tb_function(name,creatE_time) values(‘vxfqrt2adafz‘,‘2020-07-01 21:30:00‘); Query OK, 1 row affected (0.01 sec) mysql> insert into tb_function(name,creatE_time) values(‘etxzwrwbdhegqgaheqhag‘,‘2020-07-02 01:30:00‘); Query OK, 1 row affected (0.01 sec) mysql> insert into tb_function(name,creatE_time) values(‘awrs433fsgvsfwtwg‘,‘2020-07-02 03:30:00‘); Query OK, 1 row affected (0.00 sec) mysql> insert into tb_function(name,creatE_time) values(‘awrs433fsgvsfwtwg‘,‘2020-07-02 07:32:00‘); Query OK, 1 row affected (0.00 sec) mysql> insert into tb_function(name,creatE_time) values(‘awrs433fsgvsfwtwg‘,‘2020-07-02 10:32:00‘); Query OK, 1 row affected (0.00 sec) mysql> insert into tb_function(name,creatE_time) values(‘tuilklmdadq‘,‘2020-07-02 15:32:00‘); Query OK, 1 row affected (0.00 sec) mysql> insert into tb_function(name,creatE_time) values(‘wesv2wqdshehq‘,‘2020-07-02 20:32:00‘); Query OK, 1 row affected (0.00 sec) mysql> insert into tb_function(name,creatE_time) values(‘89yoijnlkwr1‘,‘2020-07-03 02:56:00‘); Query OK, 1 row affected (0.00 sec) mysql> insert into tb_function(name,creatE_time) values(‘olj;nsaaq‘,‘2020-07-03 08:41:00‘); Query OK, 1 row affected (0.01 sec) mysql> insert into tb_function(name,creatE_time) values(‘ygo;jkdsaq‘,‘2020-07-03 16:20:00‘); Query OK, 1 row affected (0.01 sec) mysql> select * from tb_function; +----+-----------------------+---------------------+ | id | name | create_time | +----+-----------------------+---------------------+ | 1 | anniuadaOAIFAPUHIA | 2020-07-01 12:00:00 | | 2 | CWQSsar3qcssg | 2020-07-01 15:00:00 | | 3 | vxfqrt2adafz | 2020-07-01 21:30:00 | | 4 | etxzwrwbdhegqgaheqhag | 2020-07-02 01:30:00 | | 5 | awrs433fsgvsfwtwg | 2020-07-02 03:30:00 | | 6 | awrs433fsgvsfwtwg | 2020-07-02 07:32:00 | | 7 | awrs433fsgvsfwtwg | 2020-07-02 10:32:00 | | 8 | tuilklmdadq | 2020-07-02 15:32:00 | | 9 | wesv2wqdshehq | 2020-07-02 20:32:00 | | 10 | 89yoijnlkwr1 | 2020-07-03 02:56:00 | | 11 | olj;nsaaq | 2020-07-03 08:41:00 | | 12 | ygo;jkdsaq | 2020-07-03 16:20:00 | +----+-----------------------+---------------------+ 12 rows in set (0.00 sec)

1.2  创建索引

在create_time字段上创建索引

mysql> alter  table tb_function add key idx_create_time(create_time);
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

1.3  按时间查询

查询创建时间是2020-07-01那天的所有记录

mysql> select  *  from  tb_function  where   date(create_time)=‘2020-07-01‘;
+----+--------------------+---------------------+
| id | name               | create_time         |
+----+--------------------+---------------------+
|  1 | anniuadaOAIFAPUHIA | 2020-07-01 12:00:00 |
|  2 | CWQSsar3qcssg      | 2020-07-01 15:00:00 |
|  3 | vxfqrt2adafz       | 2020-07-01 21:30:00 |
+----+--------------------+---------------------+
3 rows in set (0.00 sec)

执行计划如下：

mysql> explain select  *  from  tb_function  where   date(create_time)=‘2020-07-01‘;
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table       | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_function | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   12 |   100.00 | Using where |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

执行计划中可以看出是进行了全面扫描

1.4  优化

因MySQL5.7不支持函数索引，所以需要修改SQL写法来实现走索引（或者使用虚拟列的方式），上述SQL可以修改为

mysql> select  *  from  tb_function  where   create_time>=‘2020-07-01‘ and create_time<date_add(‘2020-07-01‘,INTERVAL 1 day);
+----+--------------------+---------------------+
| id | name               | create_time         |
+----+--------------------+---------------------+
|  1 | anniuadaOAIFAPUHIA | 2020-07-01 12:00:00 |
|  2 | CWQSsar3qcssg      | 2020-07-01 15:00:00 |
|  3 | vxfqrt2adafz       | 2020-07-01 21:30:00 |
+----+--------------------+---------------------+
3 rows in set (0.00 sec)

执行计划如下：

mysql> explain select  *  from  tb_function  where   create_time>=‘2020-07-01‘ and create_time<date_add(‘2020-07-01‘,INTERVAL 1 day);
+----+-------------+-------------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+
| id | select_type | table       | partitions | type  | possible_keys   | key             | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_function | NULL       | range | idx_create_time | idx_create_time | 6       | NULL |    3 |   100.00 | Using index condition |
+----+-------------+-------------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

可见，修改后，使用了索引。

2、 MySQL8.0

MySQL8.0的索引特性增加了函数索引。其实MySQL5.7中推出了虚拟列的功能，而MySQL8.0的函数索引也是依据虚拟列来实现的。将上述的案例在MySQL8.0中实现情况如下文所述。

2.1  创建函数索引

在将上述的表及数据在MySQL8.0的实例上创建，然后创建create_time的函数索引，SQL如下

mysql> alter  table tb_function add key idx_create_time((date(create_time))); --   注意里面字段的括号
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

2.2  按时间查询

mysql> select  *  from  tb_function  where   date(create_time)=‘2020-07-01‘;
+----+--------------------+---------------------+
| id | name               | create_time         |
+----+--------------------+---------------------+
|  1 | anniuadaOAIFAPUHIA | 2020-07-01 12:00:00 |
|  2 | CWQSsar3qcssg      | 2020-07-01 15:00:00 |
|  3 | vxfqrt2adafz       | 2020-07-01 21:30:00 |
+----+--------------------+---------------------+
3 rows in set (0.00 sec)

执行计划如下

mysql> explain select  *  from  tb_function  where   date(create_time)=‘2020-07-01‘;
+----+-------------+-------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| id | select_type | table       | partitions | type | possible_keys   | key             | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_function | NULL       | ref  | idx_create_time | idx_create_time | 4       | const |    3 |   100.00 | NULL  |
+----+-------------+-------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

可见，在MySQL8.0 创建对应的函数索引后，不改变SQL写法的前提下，查询的列上进行对应的函数计算后也可以走索引。

关于MySQL函数索引的优化及MySQL8.0函数索引还可以有更多的场景进行测试，建议大家多动手试试，提高SQL改写及优化的能力。

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MySQL函数索引及优化

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