title: mysql query taking too long to execute
body: <p>I have a query that is taking way too long to execute (4 seconds) even though all the fields i am querying against are indexed. Below are the query and the explain results. Any ideas what the problem is? (mysql CPU usage shoots up to 100% when executing the query</p>
  
  <pre><code>EXPLAIN SELECT count(hd.did) as NumPo, `hd`.`sid`, `src`.`Name`
  FROM (`hd`)
  JOIN `result` ON `result`.`did` = `hd`.`did`
  JOIN `sf` ON `sf`.`fid` = `hd`.`fid`
  JOIN `src` ON `src`.`sid` = `hd`.`sid`
  WHERE `sf`.`tid` =  2
  AND `result`.`set` =  'xxxxxxx'
  GROUP BY `hd`.`sid`
  ORDER BY `NumPo` DESC
  LIMIT 10;
  
  +----+-------------+--------------+--------+-------------------------+---------+---------+--------------------------+------+----------------------------------------------+
  | id | select_type | table        | type   | possible_keys           | key     | key_len | ref                      | rows | Extra                                        |
  +----+-------------+--------------+--------+-------------------------+---------+---------+--------------------------+------+----------------------------------------------+
  |  1 | SIMPLE      | sf           | ref    | PRIMARY,type            | type    | 2       | const                    |    4 | Using index; Using temporary; Using filesort |
  |  1 | SIMPLE      | hd           | ref    | PRIMARY,sid,fid         | FeedID  | 4       | f2.sf.fid                |    3 |                                              |
  |  1 | SIMPLE      | result       | ALL    | resultset               | NULL    | NULL    | NULL                     | 5322 | Using where; Using join buffer               |
  |  1 | SIMPLE      | src          | eq_ref | PRIMARY                 | PRIMARY | 4       | f2.hd.sid                |    1 |                                              |
  +----+-------------+--------------+--------+-------------------------+---------+---------+--------------------------+------+----------------------------------------------+
  </code></pre>