EXPLAIN
(select c1 from t1 group by c1,c2 order by c1 limit 1);
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using filesort
EXPLAIN
select (select c1 from t1 group by c1,c2 order by c1 limit 1) as x;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2