复合索引前导列特性
在有些文章中也称之为【索引的最佳左前缀特性】
叫什么不重要,重要的是要理解他,会去运用他----柳峰老师
【重点】
创建一个复合索引
create index idx_name_salary_dept on employee(name,salary,dept);
查询数据
mysql> explain select * from employee where name='墨菲'\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: employee
partitions: NULL
type: ref
possible_keys: idx_name_salary_dept
key: idx_name_salary_dept
key_len: 123
ref: const
rows: 1
filtered: 100.00
Extra: NULL
1 row in set, 1 warning (0.13 sec)
ERROR:
No query specified
mysql> explain select * from employee where name='墨菲' and salary='8800'\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: employee
partitions: NULL
type: ref
possible_keys: idx_name_salary_dept
key: idx_name_salary_dept
key_len: 128
ref: const,const
rows: 1
filtered: 100.00
Extra: NULL
1 row in set, 1 warning (0.30 sec)
ERROR:
No query specified
查看name和dept是否有用到索引
explain select * from employee where name='墨菲' and dept='部门A'\G;
mysql> explain select * from employee where name='墨菲' and dept='部门A'\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: employee
partitions: NULL
type: ref
possible_keys: idx_name_salary_dept
key: idx_name_salary_dept
key_len: 123
ref: const
rows: 1
filtered: 10.00
Extra: Using index condition
1 row in set, 1 warning (0.05 sec)
ERROR:
No query specified
结果是一样的,也有用到索引
如果仅仅是部门A的情况,我们发现并没有用到索引
mysql> explain select * from employee where dept='部门A'\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: employee
partitions: NULL
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 13
filtered: 10.00
Extra: Using where
1 row in set, 1 warning (0.00 sec)
ERROR:
No query specified