title: Mysql using filesort depends on LIMIT
body: <p>I have this query:</p>
  
  <pre><code>SELECT * 
  FROM `content` AS `c` 
  WHERE c.module = 20 
    AND c.categoryID 
      IN ('50', '31', '16', '10') 
  ORDER BY `c`.`released` DESC 
  LIMIT 5
  </code></pre>
  
  <p>...and I have two machines. On first machine EXPLAIN query returns 'Using where' for extra field when LIMIT is less than 3. When LIMIT is greater then 3 it returns 'Using where; Using filesort' (filesort is run for limit greater than 3).</p>
  
  <p>On the other machine, it is different for the same DB and same query: when LIMIT is greater than 60 than filesort is run.</p>
  
  <p>On both machines, my.cnf are the same!!!</p>
  
  <p>Does anyone know how using filesort depends on LIMIT and how this dependency can be changed?</p>