title: Mysql Order by clause using "FileSort"
body: <p>I have a table structure like</p>
  
  <pre><code>comment_id primary key
  comment_content 
  comment_author
  comment_author_url
  </code></pre>
  
  <p>When I fire query like </p>
  
  <pre><code>explain SELECT * FROM comments  ORDER BY comment_id
  </code></pre>
  
  <p>It outputs the results as</p>
  
  <pre><code>id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
  1   SIMPLE  comments    ALL     NULL    NULL    NULL    NULL    22563   Using filesort
  </code></pre>
  
  <p>Why is not able to find the index that I have defined as Primary Key?</p>