mysql> explain select * from test_order where a > 100 group by a;
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | test_order | NULL | index | idx_a_b_c | idx_a_b_c | 266 | NULL | 9927 | 49.99 | Using where |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+

走了索引，怎么还提示using where  ？？

create table `test_order` (
`id` int not null auto_increment comment 'id',
`a` int unsigned not null default '0',
`b` int unsigned not null default '0',
`c` varchar(64) not null default '',
`d` int default null,
primary key (`id`),
key `idx_a_b_c` (`a`,`b`,`c`),
key `idx_d` (`d`)
)

表中有10000条数据，