关于sql中有分组子查询时外层表的索引使用问题

Question

查询1

explain select * from t1 where mobile in (select mobile from t2 group by mobile, draw_date );
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------------------+
| id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows | Extra                                |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------------------+
|  1 | SIMPLE      | t2    | index | NULL          | mobile | 27      | NULL |   15 | Using index; Start temporary         |
|  1 | SIMPLE      | t1    | ref   | mobile        | mobile | 63      | func |    1 | Using index condition; End temporary |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------------------+

此时外层表t1可以用到索引

查询2

explain select * from t1 where mobile in (select mobile from t2 group by mobile, draw_date having count(*)>1);
+----+--------------------+-------+-------+---------------+--------+---------+------+------+-------------+
| id | select_type        | table | type  | possible_keys | key    | key_len | ref  | rows | Extra       |
+----+--------------------+-------+-------+---------------+--------+---------+------+------+-------------+
|  1 | PRIMARY            | t1    | ALL   | NULL          | NULL   | NULL    | NULL |   11 | Using where |
|  2 | DEPENDENT SUBQUERY | t2    | index | mobile        | mobile | 27      | NULL |   15 | Using index |
+----+--------------------+-------+-------+---------------+--------+---------+------+------+-------------+

子查询增加了having count(*)>1 外层表t1就用不上索引了。

查询3

explain select * from t1 where mobile in (select mobile from (select mobile from t2 group by mobile, draw_date having count(*)>1) a);
+----+-------------+------------+-------+---------------+--------+---------+------+------+--------------------------------------+
| id | select_type | table      | type  | possible_keys | key    | key_len | ref  | rows | Extra                                |
+----+-------------+------------+-------+---------------+--------+---------+------+------+--------------------------------------+
|  1 | PRIMARY     | <derived3> | ALL   | NULL          | NULL   | NULL    | NULL |   15 | Start temporary                      |
|  1 | PRIMARY     | t1         | ref   | mobile        | mobile | 63      | func |    1 | Using index condition; End temporary |
|  3 | DERIVED     | t2         | index | mobile        | mobile | 27      | NULL |   15 | Using index                          |
+----+-------------+------------+-------+---------------+--------+---------+------+------+--------------------------------------+

子查询再包了一层 外层表t1又可以用索引了。

怎么外层表一会能用上索引， 一会儿又不能？ 何解？