Explain 語法

# 語法
explain + DQL語句

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

# 查詢中國和美國的資料
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 欄位

ID：查詢順序號

Table：查詢的表名

Partitions：表所使用的分割槽，如果要統計十年公司訂單的金額，可以把資料分為十個區，每一年代表一個區。這樣可以大大的提高查詢效率

Select_type 欄位

select 查詢的型別，主要是用於區別普通查詢，聯合查詢，巢狀的複雜查詢

# simple：簡單的select 查詢，查詢中不包含子查詢或者 union

# primary 和 subquery，最外層查詢被標記為 primary，內層查詢被標記為 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，根據 select 語句衍生出臨時表 tmp_students 時出現
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，連表查詢時出現
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 欄位

顯示查詢語句可能用到的索引，一個或多個，或為 NULL，不一定被查詢實際使用，僅供參考使用

Key 欄位

顯示查詢語句實際使用的索引，若為 NULL，則表示沒有使用索引

Key_len 欄位

顯示索引中使用的位元組數，可通過key_len計算查詢中使用的索引長度。在不損失精確性的情況下索引長度越短越好

Ref 欄位

顯示索引的哪一列或常量被用於查詢索引列上的值

Rows 欄位

根據表統計資訊及索引選用情況，估算出找到所需的記錄所需要讀取的行數，並不是查詢結果的行數，值越大越不好

Type 欄位

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

# range：範圍查詢
mysql> explain select * from city where countrycode ='CHN' or countrycode ='USA';
#有限制查詢到的資料在總資料的 15% 以內，超過則為全文掃描，在查詢可以使用 limit 限制在 15% 以內
mysql> explain select * from city where countrycode != 'CHN' limit 500;

# ref：精確查詢
	mysql> explain select * from city where countrycode ='CHN';

# eq_ref：使用多表聯查時會出現
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：查詢的條件，是主鍵索引或者唯一鍵索引
mysql> explain select * from city where id=1;

# system：查詢級別與 const 相同，當資料很少時出現


# null：不需要讀取資料，只需要獲取最大值或者最小值
mysql> explain select max(population) from city;

Extra 欄位