MySQL group by语句如何优化

在MySQL中，新建立一张表，该表有三个字段，分别是id，a，b，插入1000条每个字段都相等的记录，如下：

mysql> show create table t1G
*************************** 1. row ***************************
    Table: t1
Create Table: CREATE TABLE `t1` (
 `id` int(11) NOT NULL,
 `a` int(11) DEFAULT NULL,
 `b` int(11) DEFAULT NULL,
 PRIMARY KEY (`id`),
 KEY `a` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
mysql> select * from t1 limit 10;
+----+------+------+
| id | a  | b  |
+----+------+------+
| 1 |  1 |  1 |
| 2 |  2 |  2 |
| 3 |  3 |  3 |
| 4 |  4 |  4 |
| 5 |  5 |  5 |
| 6 |  6 |  6 |
| 7 |  7 |  7 |
| 8 |  8 |  8 |
| 9 |  9 |  9 |
| 10 |  10 |  10 |
+----+------+------+
10 rows in set (0.00 sec)