很多開發人員在使用MySQL時經常會在部分列上進行函數計算等，導致無法走索引，在數據量大的時候，查詢效率低下。針對此種情況本文從MySQL5.7 及MySQL8.0中分別進行不同方式的優化。

1、 MySQL5.7

MySQL5.7版本中不支持函數索引，因此 遇到函數索引的時候需要進行修改，否則即使查詢的字段上有索引，執行時也無法使用索引而進行全表掃描，數據量大的表查詢時間會比較長。具體案例如下：

1.1 創建測試表及數據

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 創建索引

在create_time字段上創建索引

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 按時間查詢

查詢創建時間是2020-07-01那天的所有記錄

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)

執行計劃如下：

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)

執行計劃中可以看出是進行了全面掃描

1.4 優化

因MySQL5.7不支持函數索引，所以需要修改SQL寫法來實現走索引（或者使用虛擬列的方式），上述SQL可以修改為

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)

執行計劃如下：

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)

可見，修改后，使用了索引。

2、MySQL8.0

MySQL8.0的索引特性增加了函數索引。其實MySQL5.7中推出了虛擬列的功能，而MySQL8.0的函數索引也是依據虛擬列來實現的。將上述的案例在MySQL8.0中實現情況如下文所述。

2.1 創建函數索引

在將上述的表及數據在MySQL8.0的實例上創建，然后創建create_time的函數索引，SQL如下

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

2.2 按時間查詢

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)

執行計劃如下

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)

可見，在MySQL8.0 創建對應的函數索引后，不改變SQL寫法的前提下，查詢的列上進行對應的函數計算后也可以走索引。

關于MySQL函數索引的優化及MySQL8.0函數索引還可以有更多的場景進行測試，建議大家多動手試試，提高SQL改寫及優化的能力。

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