MySQL8.0新特性--函数索引
很多开发人员在使用MySQL时经常会在部分列上进行函数计算等，导致无法走索引，在数据量大的时候，查询效率低下。针对此种情况本文从MySQL5.7 及MySQL8.0中分别进行不同方式的优化。
1、 MySQL5.7
MySQL5.7版本中不支持函数索引，因此 遇到函数索引的时候需要进行修改，否则即使查询的字段上有索引，执行时也无法使用索引而进行全表扫描，数据量大的表查询时间会比较长。具体案例如下：
1.1 创建测试表及数据
mysql> use testdb;Database changedmysql> 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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