Sorting over multiple JOINS

[Günther Schmidt]

Hi people,
I have a performanceproblem with sorting over more than one table.
This are my example tables:

main
ID | thingID | info1 | info2

Index: PRIMARY (ID), INDEX (thingID , info1)
All columns are unsigned INT
thingID is foreign key to table

thing:
ID | thing1 | thing2 | thing3

Index: PRIMARY (ID)
All columns are unsigned INT

Now I do a JOIN:

SELECT main.info2, thing.thing1, thing.thing2, thing.thing3
FROM main
INNER JOIN thing ON main.thingID=thing.ID
WHERE main.info1 = 12

This works like a charm on our MySLQ 5.5 InnoDB tables. Resultset is about 500.000 rows with a duration of <1s.
But if I add some sorting...

ORDER BY thing.thing1, main.info2, thing.thing2

then a filesort is done because there is no index usable. It takes around 250s to get the results.
How can I change my indexes or the query to prevent the filesort? I did quite some research on that but all examples I found do not mix columns from more than one table for sorting.
What do you think?

Regards
Günther