Query optimization

[Jan Kowalski]

I have such query:

SELECT post_modified_gmt
FROM wp_posts
WHERE post_status = 'publish'
AND post_type
IN (
'post', 'page', 'attachment', 'topic'
)
ORDER BY post_date DESC
LIMIT 1

there is composite index on post_type,post_status,post_date columns but mysql uses filesort which dramaticly slows down this query execution. Is there any way to speed up this query?