MySQL using filesort :(

[Jason Hamilton]

I'm having a problem with MySQL and ORDER BY.

With this statement, extra = "Using where"

EXPLAIN SELECT * FROM c1 LEFT JOIN c2 ON c1.id = c2.id WHERE c1.id = '21'

Simply adding "ORDER BY c1.size DESC" changes the extra
to "Using where; Using filesort".

Naturally I have an index on c1.id (c2.id also has an index), and
c1.size.

I'm at a loss as to why it's going to filesort. The query speed
is quick on most results, except for when there is > ~10,000 matches,
then it can take several seconds to process. Most matches are
under 1,000 results, though some can be as high as 150,000.

Is there anything I can do to remove use of filesort, or increase
the speed of the query on large matches?

Thanks,

Jason