mysql> explain SELECT SUM(bytes/1000000) AS bytes FROM user_data WHERE uid='1740
3' AND timestamp BETWEEN '1189348200' AND '1191940199';
+----+-------------+-----------+-------+---------------------------------+---------+---------+------+------+-------------+
| id | select_type | table     | type  | possible_keys                   | key     | key_len | ref  | rows | Extra       |
+----+-------------+-----------+-------+---------------------------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | user_data | range | uidtime,idx_uid,idx_uiddaymonth | uidtime |       8 | NULL | 2328 | Using where |
+----+-------------+-----------+-------+---------------------------------+---------+---------+------+------+-------------+
1 row in set (4.81 sec)

mysql>  SELECT SUM(bytes/1000000) AS bytes FROM user_data WHERE uid='17403' AND timestamp BETWEEN '1189348200' AND '1191940199';
+---------+
| bytes   |
+---------+
| 4404.01 |
+---------+
1 row in set (16.92 sec)

mysql> show create table user_data;

CREATE TABLE `user_data` (
  `id` int(255) unsigned NOT NULL auto_increment,
  `uid` int(255) unsigned NOT NULL default '0',
  `ip` varchar(16) NOT NULL default '',
  `DAY` int(2) unsigned NOT NULL default '0',
  `MONTH` int(2) unsigned NOT NULL default '0',
  `year` int(4) unsigned NOT NULL default '0',
  `bytes` bigint(255) unsigned NOT NULL default '0',
  `bytes_local` bigint(20) unsigned NOT NULL default '0',
  `timestamp` int(255) unsigned NOT NULL default '0',
  PRIMARY KEY  (`id`),
  KEY `uidtime` (`uid`,`timestamp`),
  KEY `idx_uid` (`uid`),
  KEY `idx_uiddaymonth` (`uid`,`DAY`,`MONTH`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 MAX_ROWS=300000000

whilst running the select
data# iostat 2
      tty           aacd0              fd0              md0             cpu
 tin tout  KB/t tps  MB/s   KB/t tps  MB/s   KB/t tps  MB/s  us ni sy in id
   0   44  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00   7  2  5  0 87
   0   77 16.00 125  1.95   0.00   0  0.00   0.00   0  0.00   0  0  0  0 100
   0   77 16.00 128  2.00   0.00   0  0.00   0.00   0  0.00   0  0  0  0 100
   0   77 16.00 138  2.15   0.00   0  0.00   0.00   0  0.00   0  0  0  0 100
   0   77 16.00 126  1.96   0.00   0  0.00   0.00   0  0.00   0  0  0  1 99
   0   76 16.00 132  2.06   0.00   0  0.00   0.00   0  0.00   0  0  0  0 100
   0   77 16.00 120  1.87   0.00   0  0.00   0.00   0  0.00   0  0  1  0 99
   0  176 16.00 130  2.03   0.00   0  0.00   0.00   0  0.00   1  0  2  0 98
   0   76  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00   0  0  0  0 100
   0   77  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00   0  0  0  0 100
   0   77  0.00   0  0.00   0.00   0  0.00   0.00   0  0.00   0  0  0  0 100
^C

mysql> show table status like 'user_data'\G
*************************** 1. row ***************************
           Name: user_data
         Engine: MyISAM
        Version: 9
     Row_format: Dynamic
           Rows: 85393210
 Avg_row_length: 51
    Data_length: 4411811788
Max_data_length: 1099511627775
   Index_length: 4213731328
      Data_free: 14313116
 Auto_increment: 293588813
    Create_time: 2007-09-23 12:28:00
    Update_time: 2007-10-04 19:11:18
     Check_time: 2007-10-04 15:52:57
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options: max_rows=300000000
        Comment:
1 row in set (0.02 sec)