-- Why would this (sub)query do a filesort?  It's already got an index on activity_reviewid!
 
--- First query: 
EXPLAIN SELECT a.activity_reviewid
,MIN(a.activity_startsecs) startsecs
, MAX(a.activity_startsecs + a.activity_durationsecs) endsecs
,SUM(a.activity_durationsecs) totalsecs
,SUM(IF((a.activity_code)=('R'),a.activity_durationsecs,0)) reviewersecs
,SUM(IF((a.activity_code)=('A'),a.activity_durationsecs,0)) authorsecs 
FROM activity a 
GROUP BY a.activity_reviewid
;
 
 
+----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra                           |
+----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+
|  1 | SIMPLE      | a     | ALL  | NULL          | NULL | NULL    | NULL | 21453 | Using temporary; Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+
 
 
CREATE TABLE `activity` (
  `activity_id` int(11) NOT NULL AUTO_INCREMENT,
  `activity_startdate` datetime NOT NULL,
  `activity_durationsecs` int(10) unsigned NOT NULL,
  `activity_userid` int(11) NOT NULL,
  `activity_reviewid` int(11) NOT NULL,
  `activity_code` char(1) NOT NULL,
  `activity_startsecs` int(10) unsigned NOT NULL,
  PRIMARY KEY (`activity_id`),
  KEY `idx_activity_reviewId` (`activity_reviewid`),
  KEY `idx_activity_startdate` (`activity_startdate`),
  KEY `idx_activity_userId_startDate` (`activity_userid`,`activity_startdate`),
  KEY `idx_activity_userId_reviewId` (`activity_userid`,`activity_reviewid`)
) ENGINE=MyISAM AUTO_INCREMENT=21472 DEFAULT CHARSET=utf8