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)
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