急！高分求助！
假设一张表a分别有a1,a2,a3三个字段，a1是主键     ，a2是索引，a3是内容。a1,a2是类型是int,a3任意。现在想做到如下例子效果：    
 
a1         a2         a3    
1             1             A    
2             2             B    
3             2             C    
4             2             D    
5             3             E    
6             3             F    
7             4             G    
 
设计一个数据库语句，使得如果limit     2的话，得到a1=7和a1=5的数据，如果limit     3的话，则得到a1=7，a1=5和a1=2的数据，因为该a表的记录总数有快200万行，所以希望各位能提供一个高效的解决该问题的数据库语句，先谢谢各位！

------解决方案--------------------
select min(a1),a2,a3 from csdn group by a2 order by a1 desc limit 3;
------解决方案--------------------
mysql> create table t1
-> (
-> a1 int not null auto_increment primary key,
-> a2 int not null,
-> a3 char(1) not null
-> );
Query OK, 0 rows affected (0.09 sec)

mysql> create index t_a2 on t1(a2);
Query OK, 0 rows affected (0.19 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> insert into t1 (a2,a3) values
-> (1, 'A '),
-> (2, 'B '),
-> (2, 'C '),
-> (2, 'D '),
-> (3, 'E '),
-> (3, 'F '),
-> (4, 'G ');
Query OK, 7 rows affected (0.16 sec)
Records: 7 Duplicates: 0 Warnings: 0

mysql> select * from t1 group by a2 order by a1 desc limit 2;
+----+----+----+
| a1 | a2 | a3 |
+----+----+----+
| 7 | 4 | G |
| 5 | 3 | E |
+----+----+----+
2 rows in set (0.00 sec)

mysql> select * from t1 group by a2 order by a1 desc limit 3;
+----+----+----+
| a1 | a2 | a3 |
+----+----+----+
| 7 | 4 | G |
| 5 | 3 | E |
| 2 | 2 | B |
+----+----+----+
3 rows in set (0.00 sec)