Optimal indexing for SELECT (mysql)
SUGGESTION #1
Based on the WHERE clause, I highly recommend a compound index
ALTER TABLE mytable ADD INDEX (md5,created_at);

That way, all parts of the WHERE clause are answered by the index.
SUGGESTION #2
Since you are imposing a limit of 200, collect the 200 keys first, then join the keys
SELECT B.* FROM
(
    SELECT id FROM mytable
    WHERE md5 = ?
    AND created_at &gt;= ?
    AND created_at &lt;= ? LIMIT 200
) A INNER JOIN mytable B USING (id);

I have suggested this technique before in StackOverflow
GIVE IT A TRY !!!