共有两个表格，其结构如下：

mysql> desc product;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| brand | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.02 sec)
mysql> desc sales;
+-------------+-------------+------+-----+---------+-------+
| Field       | Type        | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| id          | int(11)     | YES  |     | NULL    |       |
| yearofsales | varchar(10) | YES  |     | NULL    |       |
| price       | int(11)     | YES  |     | NULL    |       |
+-------------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

在这里 id 是外键。
查询内容如下：
1

mysql> select brand,sum(price),yearofsales 
       from product p, sales s 
       where p.id=s.id 
       group by s.id,yearofsales;
+-------+------------+-------------+
| brand | sum(price) | yearofsales |
+-------+------------+-------------+
| Nike  |  917504000 | 2012        |
| FF    |  328990720 | 2010        |
| FF    |  328990720 | 2011        |
| FF    |  723517440 | 2012        |
+-------+------------+-------------+
4 rows in set (1.91 sec)

2

mysql> select brand,tmp.yearofsales,tmp.sum 
       from product p 
       join (
           select id,yearofsales,sum(price) as sum
           from sales
           group by yearofsales,id
       ) tmp on p.id=tmp.id ;
+-------+-------------+-----------+
| brand | yearofsales | sum       |
+-------+-------------+-----------+
| Nike  | 2012        | 917504000 |
| FF    | 2011        | 328990720 |
| FF    | 2012        | 723517440 |
| FF    | 2010        | 328990720 |
+-------+-------------+-----------+
4 rows in set (1.59 sec)

问题是：为什么第二个查询比第一个查询花费更少的时间？我也以不同的顺序执行过多次。