SELECT *
FROM (
SELECT DISTINCT meta_value
FROM postdata pd
WHERE pd.meta_key = 'date'
) pd
JOIN post p
ON p.id =
(
SELECT post_id
FROM postdata pdi
JOIN post pi
ON pi.id = pdi.post_id
WHERE pdi.meta_key = 'date'
AND pdi.meta_value = pd.meta_value
ORDER BY
pi.post_date DESC, pi.id DESC
LIMIT 1
)