simple uses of order by datetime keep causing 'using filesort'

[park albert]

Hello,
I have a simple table, InnoDB,

idx => primary key, auto-increment,
name, ....

start_datetime => index

and i tried a simple query like,

"explain select * from start_datetime_info order by start_datetime"
i tried it with test data consisting of more than 100,000 rows.
it always shows 'using filesort'.

i thought it would be very simple. i indexed 'start_datetime' column and tried 'order by start_datetime'

what kind of mistakes could i make on this..