To by mohlo fungovat, i když jsem nemohl říct moc o jeho výkonu...
SELECT DATE(FROM_UNIXTIME(MyTimestamp)) AS ForDate, *
FROM MyPostsTable
WHERE 2 >=
(
SELECT COUNT(*)
FROM MyPostsTable AS lookup
WHERE DATE(FROM_UNIXTIME(lookup.MyTimestamp)) = DATE(FROM_UNIXTIME(MyPostsTable.MyTimestamp))
AND lookup.MyTimeStamp >= MyPostsTable.MyTimestamp
)