mysql> explain select * from test_order where a = 100 and b > 200 group by b;
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | test_order | NULL | range | idx_a_b_c | idx_a_b_c | 8 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

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

select * ,不是select 索引字段， 怎么还提示索引全覆盖？？