mysql> explain SELECT SQL_NO_CACHE *
    -> FROM
    ->   blog
    ->   join
    ->   (
    ->   SELECT max(dateline) maxdateline
    ->        , blog.userid userid
    ->   FROM
    ->     blog
    ->   GROUP BY
    ->     blog.userid
    ->   ORDER BY
    ->     max(dateline) DESC
    ->   LIMIT
    ->     250
    ->   ) grouper
    ->   on (  blog.dateline = grouper.maxdateline and blog.userid = grouper.userid);
+----+-------------+------------+-------+-----------------+----------+---------+------------------------------------+------+----------------------------------------------+
| id | select_type | table      | type  | possible_keys   | key      | key_len | ref                                | rows | Extra                                        |
+----+-------------+------------+-------+-----------------+----------+---------+------------------------------------+------+----------------------------------------------+
|  1 | PRIMARY     | <derived2> | ALL   | NULL            | NULL     | NULL    | NULL                               |  250 |                                              |
|  1 | PRIMARY     | blog       | ref   | dateline,userid | dateline | 8       | grouper.maxdateline,grouper.userid |    1 |                                              |
|  2 | DERIVED     | blog       | index | NULL            | userid   | 9       | NULL                               | 1054 | Using index; Using temporary; Using filesort |
+----+-------------+------------+-------+-----------------+----------+---------+------------------------------------+------+----------------------------------------------+
3 rows in set (0.00 sec)

mysql> explain select t1.* from blog t1
    ->   where t1.dateline= (
    ->    SELECT max(t2.dateline) from blog t2 where t2.userid=t1.userid
    ->   )
    ->   order by t1.dateline desc
    ->   limit 250;
+----+--------------------+-------+-------+---------------+----------+---------+-------------------+-------+-------------+
| id | select_type        | table | type  | possible_keys | key      | key_len | ref               | rows  | Extra       |
+----+--------------------+-------+-------+---------------+----------+---------+-------------------+-------+-------------+
|  1 | PRIMARY            | t1    | index | NULL          | dateline | 9       | NULL              | 15804 | Using where |
|  2 | DEPENDENT SUBQUERY | t2    | ref   | userid        | userid   | 4       | uantona.t1.userid |    15 | Using index |
+----+--------------------+-------+-------+---------------+----------+---------+-------------------+-------+-------------+
2 rows in set (0.00 sec)