Explain 索引优化分析

Explain 语法

# 语法
explain + DQL语句

mysql> explain select * from city where countrycode =‘CHN‘ or countrycode =‘USA‘;

# 查询中国和美国的数据
mysql> select * from city where countrycode =‘CHN‘ or countrycode =‘USA‘;
mysql> select * from city where countrycode in (‘CHN‘,‘USA‘);
mysql> select * from city where countrycode = ‘CHN‘ union all select * from city where countrycode = ‘USA‘;

# Explain 分析 SQL 
mysql>  explain select * from city where countrycode =‘CHN‘ or countrycode =‘USA‘;
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+
| id | select_type | table | type  | possible_keys | key         | key_len | ref  | rows | Extra                 |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | city  | range | CountryCode   | CountryCode | 3       | NULL |  637 | Using index condition |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

ID，Table，Partitions 字段

ID：查询顺序号

Table：查询的表名

Partitions：表所使用的分区，如果要统计十年公司订单的金额，可以把数据分为十个区，每一年代表一个区。这样可以大大的提高查询效率

Select_type 字段

select 查询的类型，主要是用于区别普通查询，联合查询，嵌套的复杂查询

# simple：简单的select 查询，查询中不包含子查询或者 union

# primary 和 subquery，最外层查询被标记为 primary，内层查询被标记为 subquery
mysql> explain select name,countrycode,district,population from city where population=(select max(population) from city);
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | city  | ALL  | NULL          | NULL | NULL    | NULL | 4188 | Using where |
|  2 | SUBQUERY    | city  | ALL  | NULL          | NULL | NULL    | NULL | 4188 | NULL        |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
2 rows in set (0.00 sec)



# derived，根据 select 语句衍生出临时表 tmp_students 时出现
mysql> select *  from (select *  from students_backup) tmp_tudents  where id=2;
+----+------+-----+--------+---------------------+----------+-------+
| id | name | age | gender | register_time       | hobby    | phone |
+----+------+-----+--------+---------------------+----------+-------+
|  2 | wzh  |  18 | M      | 2020-07-14 22:02:04 | wzhhobby | 12    |
+----+------+-----+--------+---------------------+----------+-------+
1 row in set (0.00 sec)

mysql> explain select *  from (select *  from students_backup) tmp_tudents  where id=2;
+----+-------------+-----------------+------+---------------+-------------+---------+-------+------+-------+
| id | select_type | table           | type | possible_keys | key         | key_len | ref   | rows | Extra |
+----+-------------+-----------------+------+---------------+-------------+---------+-------+------+-------+
|  1 | PRIMARY     | <derived2>      | ref  | <auto_key0>   | <auto_key0> | 4       | const |    0 | NULL  |
|  2 | DERIVED     | students_backup | ALL  | NULL          | NULL        | NULL    | NULL  |    7 | NULL  |
+----+-------------+-----------------+------+---------------+-------------+---------+-------+------+-------+
2 rows in set (0.00 sec)




# union  和  union result，连表查询时出现
mysql> explain select * from students where id> 3 union select * from students_backup where id >3;
+----+--------------+-----------------+-------+---------------+---------+---------+------+------+-----------------+
| id | select_type  | table           | type  | possible_keys | key     | key_len | ref  | rows | Extra           |
+----+--------------+-----------------+-------+---------------+---------+---------+------+------+-----------------+
|  1 | PRIMARY      | students        | range | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using where     |
|  2 | UNION        | students_backup | range | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using where     |
| NULL | UNION RESULT | <union1,2>      | ALL   | NULL          | NULL    | NULL    | NULL | NULL | Using temporary |
+----+--------------+-----------------+-------+---------------+---------+---------+------+------+-----------------+
3 rows in set (0.00 sec)

Possible_keys 字段

显示查询语句可能用到的索引，一个或多个，或为 NULL，不一定被查询实际使用，仅供参考使用

Key 字段

显示查询语句实际使用的索引，若为 NULL，则表示没有使用索引

Key_len 字段

显示索引中使用的字节数，可通过key_len计算查询中使用的索引长度。在不损失精确性的情况下索引长度越短越好

Ref 字段

显示索引的哪一列或常量被用于查找索引列上的值

Rows 字段

根据表统计信息及索引选用情况，估算出找到所需的记录所需要读取的行数，并不是查询结果的行数，值越大越不好

Type 字段

# index：扫描全部索引
mysql> ALTER TABLE city ADD INDEX(name);
mysql> explain select Name from city;

# range：范围查询
mysql> explain select * from city where countrycode =‘CHN‘ or countrycode =‘USA‘;
#有限制查询到的数据在总数据的 15% 以内，超过则为全文扫描，在查询可以使用 limit 限制在 15% 以内
mysql> explain select * from city where countrycode != ‘CHN‘ limit 500;

# ref：精确查询
	mysql> explain select * from city where countrycode =‘CHN‘;

# eq_ref：使用多表联查时会出现
mysql> create table students_backup like students;
mysql> insert into students_backup select * from students;
mysql> explain select * from students,students_backup student  where student.id=students.id and student.id > 5;
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
| id | select_type | table    | type   | possible_keys | key     | key_len | ref              | rows | Extra       |
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
|  1 | SIMPLE      | students | range  | PRIMARY       | PRIMARY | 4       | NULL             |    2 | Using where |
|  1 | SIMPLE      | student  | eq_ref | PRIMARY       | PRIMARY | 4       | mydb.students.id |    1 | NULL        |
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+

# const：查询的条件，是主键索引或者唯一键索引
mysql> explain select * from city where id=1;

# system：查询级别与 const 相同，当数据很少时出现


# null：不需要读取数据，只需要获取最大值或者最小值
mysql> explain select max(population) from city;

Extra 字段

Explain 索引优化分析

原文：https://www.cnblogs.com/zzzwqh/p/13332853.html