explain select month,count(*) from domains where us > 0 group by month;
+----+-------------+-------------+------+---------------+------+---------+------+----------+----------------------------------------------+
| id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows     | Extra                                        |
+----+-------------+-------------+------+---------------+------+---------+------+----------+----------------------------------------------+
|  1 | SIMPLE      | domains | ALL  | NULL          | NULL | NULL    | NULL | 49140069 | Using where; Using temporary; Using filesort | 
+----+-------------+-------------+------+---------------+------+---------+------+----------+----------------------------------------------+
1 row in set (0.00 sec)

(root@localhost) [domains]> select month,count(*) from domains where us > 0 group by month;
+--------------+----------+
| month        | count(*) |
+--------------+----------+
| January 2007 |   121325 | 
| March 2007   |      107 | 
+--------------+----------+
2 rows in set (19.56 sec)


(root@localhost) [domains]> select month,count(*) from todo.ovt left join domains using(domain) where us > 0 ;
+--------------+----------+
| month        | count(*) |
+--------------+----------+
| January 2007 |   121325 | 
| March 2007   |   310485 | 
+--------------+----------+
2 rows in set (55.36 sec)