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`)
)

mysql> explain select * from test_order where a = 100 order by c\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test_order
partitions: NULL
type: ref
possible_keys: idx_a_b_c
key: idx_a_b_c
key_len: 4
ref: const
rows: 1
filtered: 100.00
Extra: Using filesort

不理解，明明走了索引，怎么还是filesort？？