三十七、【进阶】SQL的explain

1、explain

2、基础使用

在使用explain关键字时，只需要在所执行语句前加上explain即可

mysql> explain select * from stu where id=3;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | stu   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

3、id

4、select_type

5、type

 （1）当根据主键or唯一索引进行访问时，type参数会显示为const：

mysql> explain select * from stu where id=3;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | stu   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

（2）当不查询任何表时，type参数显示为NULL：

mysql> explain select 'a';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

（3）使用非唯一性索引，type参数通常显示为ref：

6、possible_key

显示这张表上可能用到的索引，一个或者多个

7、Key

实际使用的索引，如果为NULL，则没有使用索引

8、Key_len

表示索引中使用的字节数，该值为索引字段最大可能长度，并非实际使用长度，在不损失精确性的前提下，长度越短越好

9、rows

MySQL认为必须要执行查询的行数，在innodb引擎的表中，是一个估计值，可能并不总是准确的。

10、filtered

表示返回结果的行数栈读取行数的百分比，filtered的值越大越好