索引优化实战一
作者：互联网
一 防止索引失效口诀
二 表脚本
CREATE TABLE staffs(
id INT PRIMARY KEY AUTO_INCREMENT,
`name` VARCHAR(24)NOT NULL DEFAULT'' COMMENT'姓名',
`age` INT NOT NULL DEFAULT 0 COMMENT'年龄',
`pos` VARCHAR(20) NOT NULL DEFAULT'' COMMENT'职位',
`add_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT'入职时间'
)CHARSET utf8 COMMENT'员工记录表';
INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('z3',22,'manager',NOW());
INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('July',23,'dev',NOW());
INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('2000',23,'dev',NOW());
ALTER TABLE staffs ADD INDEX index_staffs_nameAgePos(`name`,`age`,`pos`)
mysql> select * from staffs;
+----+------+-----+---------+---------------------+
| id | name | age | pos | add_time |
+----+------+-----+---------+---------------------+
| 1 | z3 | 22 | manager | 2021-09-19 09:15:14 |
| 2 | July | 23 | dev | 2021-09-19 09:15:14 |
| 3 | 2000 | 23 | dev | 2021-09-19 09:15:14 |
+----+------+-----+---------+---------------------+
3 rows in set (0.00 sec)三 全值匹配我最爱和最佳左前缀法则
1 说明
a 全值匹配我最爱
查询的字段按照顺序在索引中都可以匹配到。SQL 中查询字段的顺序，跟使用索引中字段的顺序，没有关系。优化器会在不影响 SQL 执行结果的前提下，给你自动地优化。
b 最佳左前缀法则
查询字段与索引字段顺序的不同会导致索引无法充分使用，甚至索引失效。
原因：使用复合索引，需要遵循最佳左前缀法则，即如果索引了多列，要遵守最左前缀法则。指的是查询从索引的最左前列开始并且不跳过索引中的列。
结论：过滤条件要使用索引必须按照索引建立时的顺序，依次满足，一旦跳过某个字段，索引后面的字段都无法被使用。
2 实战
mysql> show index from staffs;
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| staffs | 0 | PRIMARY | 1 | id | A | 2 | NULL | NULL | | BTREE | | |
| staffs | 1 | index_staffs_nameAgePos | 1 | name | A | 3 | NULL | NULL | | BTREE | | |
| staffs | 1 | index_staffs_nameAgePos | 2 | age | A | 3 | NULL | NULL | | BTREE | | |
| staffs | 1 | index_staffs_nameAgePos | 3 | pos | A | 3 | NULL | NULL | | BTREE | | |
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)
# 能使用上索引，索引使用了一个字段
mysql> explain select * from staffs where name = 'July';
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 74 | const | 1 | 100.00 | NULL |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
# 能使用上索引，索引使用了两个字段
mysql> explain select * from staffs where name = 'July' and age=15;
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 78 | const,const | 1 | 100.00 | NULL |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
# 能使用上索引，索引使用了三个字段
mysql> explain select * from staffs where name = 'July' and age=15 and pos='dev';
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 140 | const,const,const | 1 | 100.00 | NULL |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
# 调换 age=15 和 pos='dev' 的顺序。SQL 中查询字段的顺序，跟使用索引中字段的顺序，没有关系。优化器会在不影响 SQL 执行结果的前提下，给你自动地优化。
mysql> explain select * from staffs where name = 'July' and pos='dev' and age=15;
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 140 | const,const,const | 1 | 100.00 | NULL |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
# 索引失效
mysql> explain select * from staffs where age=15 and pos='dev';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
# 索引失效：带头大哥（name）不能死，火车不能没有火车头（name）
mysql> explain select * from staffs where pos='dev';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
# 索引部分失效，只用到了 name：中间兄弟（age）不能断————不能跳过索引中的列
mysql> explain select * from staffs where name = 'July' and pos='dev';
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | staffs | NULL | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 74 | const | 1 | 33.33 | Using index condition |
+----+-------------+--------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
标签：实战,index,NULL,name,索引,staffs,优化,select 来源： https://blog.csdn.net/chengqiuming/article/details/120377619