This SQL will delete postmeta that's still around from deleted posts.

remove_old_postmeta.sql

-- Old Post Meta sometimes creeps
-- in from manually deleting revisions
-- and other things direct from the
-- database. Using this SQL below
-- you can remove the postmeta remnants
-- that have been left behind.

DELETE a
FROM wp_postmeta AS a
LEFT JOIN wp_posts AS b
ON b.ID = a.post_id
WHERE b.ID IS NULL