Why is below 2 query generating different execution plan in MYSQL?

Only difference in the below query is the order by column. The resultset of both query is same. However, the 2nd query execution plan shows 'Using temporary; Using filesort' for ordering where as for 1st query it does not require filesort. Apart from id I have index on c.table_b_id and c.some_indexed_id SELECT * FROM TableA a inner join TableB b on a.id = b.id inner join TableC c on c.table_b_id = b.id where c.some_indexed_id = 1 order by c.table_b_id asc; SELECT * FROM TableA a inner join TableB b on a.id = b.id inner join TableC c on c.table_b_id = b.id where c.indexed_id = 1 order by b.id asc;