问题源自这篇文章：MySQL · 答疑释惑· using filesort VS using temporary

文章中：

create table t1(    
id int, col1 int, col2 varchar(10),
key(id, col1));

create table t2(
id int, col1 int, col2 varchar(10),
key(col1));

case2:

mysql> explain select * from t1 force index(id), t2 where t1.id=1 and t1.col1 = t2.col2 order by t1.col2;
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra                       |
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------------+
|  1 | SIMPLE      | t1    | ref  | id            | id   | 5       | const |    1 | Using where; Using filesort |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL  |    1 | Using where                 |
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------------+

case3:

mysql> explain select * from t1 force index(id), t2 where t1.id=1 and t1.col1 = t2.col2 order by t2.col1 ;
+----+-------------+-------+------+---------------+------+---------+-------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra                                        |
+----+-------------+-------+------+---------------+------+---------+-------+------+----------------------------------------------+
|  1 | SIMPLE      | t1    | ref  | id            | id   | 5       | const |    1 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL  |    1 | Using where; Using join buffer               |
+----+-------------+-------+------+---------------+------+---------+-------+------+----------------------------------------------+

文章结论：

case2: order by谓词，是在第一个表t1上完成，所以只需要在t1表上使用filesort，然后排序后的结果集join t2表。
case 3: order by的字段在t2表上，所以需要把t1，t2表join的结果保存到temporary表上，然后对临时表进行filesort，最后输出结果。

我的问题：

为什么case2 case3都需要做join，为什么case3需要保存到temporary表，而case2不需要呢？？
请大佬解惑。