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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