MySQL ORDER BY与EXPLAIN命令

让我们首先创建一个表-

mysql> create table DemoTable606 (Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,FirstName varchar(100));

使用插入命令在表中插入一些记录-

mysql> insert into DemoTable606(FirstName) values('John');
mysql> insert into DemoTable606(FirstName) values('Robert');
mysql> insert into DemoTable606(FirstName) values('Chris');
mysql> insert into DemoTable606(FirstName) values('David');

使用select语句显示表中的所有记录-

mysql> select *from DemoTable606;

这将产生以下输出-

+----+-----------+
| Id | FirstName |
+----+-----------+
|  1 | John      |
|  2 | Robert    |
|  3 | Chris     |
|  4 | David     |
+----+-----------+
4 rows in set (0.00 sec)

以下是使用EXPLAIN命令的查询：

mysql> explain select * from DemoTable606 order by Id;
+----+-------------+--------------+------------+-------+---------------+---------+---------+------+------+----------+-------+
| id | select_type | table        | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+---------------+---------+---------+------+------+----------+-------+
| 1  | SIMPLE      | DemoTable606 | NULL       | index | NULL          | PRIMARY | 4       | NULL | 4    | 100.00   | NULL  |
+----+-------------+--------------+------------+-------+---------------+---------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from DemoTable606 order by Id,FirstName;
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table        | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1  | SIMPLE      | DemoTable606 | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 4    | 100.00   | Using filesort |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from DemoTable606 order by Id,RAND();
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table        | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                           |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| 1  | SIMPLE      | DemoTable606 | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 4    | 100.00   | Using temporary; Using filesort |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)