我要评论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 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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