很多開發人員在使用MySQL時經常會在部分列上進行函式計算等，導致無法走索引，在資料量大的時候，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版本中不支援函式索引，因此 遇到函式索引的時候需要進行修改，否則即使查詢的欄位上有索引，執行時也無法使用索引而進行全表掃描，資料量大的表查詢時間會比較長。具體案例如下：
1.1 建立測試表及資料
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 建立索引
在create_time欄位上建立索引
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
查詢建立時間是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不支援函式索引，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，修改後，使用了索引。
2、 MySQL8.0
MySQL8.0的索引特性增加了函式索引。其實MySQL5.7中推出了虛擬列的功能，而MySQL8.0的函式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 建立函式索引
在將上述的表及資料在MySQL8.0的例項上建立，然後建立create_time的函式索引，SQL如下
mysql> alter table tb_function add key idx_create_time((date(create_time))); -- 注意裡面欄位的括號 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 建立對應的函式索引後，不改變SQL寫法的前提下，查詢的列上進行對應的函式計算後也可以走索引。
關於MySQL函式索引的優化及MySQL8.0函式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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