Mysql 索引命中级别分析

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Mysql索引命中级别

结果值从好到坏依次是：

1. system/const, system在mysql8上好像没有了。一般主键唯一结果查询时会是const。
2. eq_ref:类似ref，区别就在使用的索引是唯一索引，对于每个索引键值，表中只有一条记录匹配，简单来说，就是多表连接中使用primary key或者 unique key作为关联条件
3. ref: 查找条件列使用了索引而且不为主键和unique。其实，意思就是虽然使用了索引，但该索引列的值并不唯一，有重复。这样即使使用索引快速查找到了第一条数据，仍然不能停止，要进行目标值附近的小范围扫描。但它的好处是它并不需要扫全表，因为索引是有序的，即便有重复值，也是在一个非常小的范围内扫描。
4. range：有范围的索引扫描，相对于index的全表扫描，他有范围限制，因此要优于index
5. index:另一种形式的全表扫描，只不过他的扫描方式是按照索引的顺序
6. all:全表扫描

准备工作

创建教师表

CREATE TABLE teacher ( 
	id INT ( 5 ), 
	NAME VARCHAR ( 20 ), 
	card_id INT ( 5 ) 
);

创建教师卡信息表

CREATE TABLE teacher_card ( 
	id INT ( 5 ), 
	remark VARCHAR ( 20 ) 
);

插入教师信息

insert into teacher values(1, '张三', 1);
insert into teacher values(2, '李四', 2);
insert into teacher values(3, '王五', 3);

插入教师卡信息

insert into teacher_card values(1, '张老师的卡');
insert into teacher_card values(2, '李老师的卡');
insert into teacher_card values(3, '王老师的卡');

const类型测试

测试前添加主键索引

alter table teacher add  constraint pk_teacher_id primary key(id);

衍生表只有一条数据的主查询；衍生表通过主键查询只有一条数据，再通过这条数据进行主查询。

EXPLAIN select *  from teacher where id=1

得到执行计划结果：

+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | teacher | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

1. 仅仅能查到一条数据的SQL，用于primary key 或 unique的索引(其他索引类型不属于)。
2. 主键查询只有一条数据的情况，类型为const

eq_ref类型测试

给teacher_card添加主键

alter table teacher_card add constraint pk_teacher_card_id primary key(id);

分析对teacher表进行索引唯一查询

explain select t.card_id from teacher t, teacher_card tc where t.card_id = tc.id;
explain select t.card_id from teacher t left join teacher_card tc on t.card_id = tc.id;

得到执行计划结果

+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------------------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref                            | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------------------+------+----------+-------------+
|  1 | SIMPLE      | t     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                           |    3 |   100.00 | NULL        |
|  1 | SIMPLE      | tc    | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | mysql_ref_index_test.t.card_id |    1 |   100.00 | Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------------------+------+----------+-------------+

唯一性索引，表索引与另外表的主键进行关联，两张表之间每条数据要一一对应(每个都要一一对应，不能一个对应多个，不能没有对应)，查询的数据在表中是唯一性，不能有重复。

ref类型测试

非唯一线性索引，对于每个索引键的查询返回的数据为0或多条。

给teacher表的name的字段添加索引

alter table teacher add index index_teacher_name (name);

• 根据name = 张三查询出两条数据
• 根据name索引直接查询出来的值为ref类型。

sqlexplain select * from teacher where name = '张三';

执行计划结果：

+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys      | key                | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | teacher | NULL       | ref  | index_teacher_name | index_teacher_name | 83      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+

range类型测试

• 检查指定范围行，where后面是一个范围查询 (between、in、>、)
• 查看range类型的索引

in的情况

explain select * from teacher t where t.id in (1, 2);

执行计划结果

+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t     | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

between的情况

explain select * from teacher where id between 1 and 2;

执行计划结果

+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | teacher | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

比较情况

explain select * from teacher where id < 3;

执行计划结果

+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | teacher | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

范围查询指定的索引，其索引类型为range：

index类型测试

1. 查询索引中的所有数据。
2. 查询索引的所有数据，其中name就是索引
3. 直接走了索引，不需要回溯

explain select name from teacher;

执行计划

+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key                | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | teacher | NULL       | index | NULL          | index_teacher_name | 83      | NULL |    3 |   100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+

all类型测试

1. 查询表中的所有数据，或者根据不是索引的字段查询。
2. 也就是走全表扫描了， 不能走索引。

EXPLAIN select * from teacher;

执行计划结果

+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | teacher | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+

type类型总结：

1. system/const：结果只有一条数据。
2. eq_ref：结果多条，但是每条数据都是唯一的。
3. ref：结果多条，但是查询到的数据可以是多条，且数据可以重复。
4. range：根究索引字段进行范围查询。