MySQL:preciso mostrar de 1 ao máximo n post de cada usuário por cada dia

select post_id, user_id, post_datetime, post_text from (
  select posts.*,
    if (user_id = @prev_user and date(post_datetime) = date(@prev_day),
      @row := @row + 1, @row := 1) idx,
    @prev_user := user_id,
    @prev_day := post_datetime
  from posts, (select @row := 1, @prev_user := null, @prev_day := null) init
  order by date(post_datetime), user_id, post_datetime desc
) s
where s.idx <= 2

+---------+---------+---------------------------------+----------------+
| POST_ID | USER_ID |          POST_DATETIME          |   POST_TEXT    |
+---------+---------+---------------------------------+----------------+
|       4 |     100 | December, 01 2012 04:00:00+0000 | lorem ipsum 4  |
|       2 |     100 | December, 01 2012 02:00:00+0000 | lorem ipsum 2  |
|       3 |     101 | December, 01 2012 03:00:00+0000 | lorem ipsum 3  |
|       5 |     102 | December, 01 2012 05:00:00+0000 | lorem ipsum 5  |
|       6 |     100 | December, 02 2012 03:00:00+0000 | lorem ipsum 6  |
|      10 |     101 | December, 02 2012 07:00:00+0000 | lorem ipsum 10 |
|       9 |     101 | December, 02 2012 06:00:00+0000 | lorem ipsum 9  |
|       7 |     102 | December, 02 2012 04:00:00+0000 | lorem ipsum 7  |
+---------+---------+---------------------------------+----------------+