create table t1 (c1 int not null, c2 int, unique index(c1), index (c2)) engine=innodb;
insert into t1 (c1, c2) select 1, round(rand()*100);
insert into t1 (c1, c2) select (select max(c1) from t1) + c1, c1*93563%100 from t1;
insert into t1 (c1, c2) select (select max(c1) from t1) + c1, c1*93563%100 from t1;
insert into t1 (c1, c2) select (select max(c1) from t1) + c1, c1*93563%100 from t1;
select count(*) from t1;
count(*)
8
explain select * from t1 where c2 = 1 order by c1;
id	1
select_type	SIMPLE
table	t1
type	ref
possible_keys	c2
key	c2
key_len	5
ref	const
rows	1
Extra	Using where; Using index
drop table t1;