title: Threaded comment system nested set - multiple roots?
body: <p>I'm implementing a threaded comment system for topics ie there is a topic and then threaded comments on each topic.</p>
  
  <p>Nested set 'seems' like the popular way to go, but how should I implement the roots of each thread?  For example:</p>
  
  <ul>
  <li><p>the comments could be one massive nested set.  I may be wrong but it seems like it would be slower all as one tree.</p></li>
  <li><p>the comments could have one root thread for each topic.  But then it would seem like I would have to make a blank root for the each topic, having blank roots just seems odd.</p></li>
  <li><p>each first level comment could be a root.  This eliminates the blank root but seems like there would be a ton of root threads and say to render a page with 50 first level commments would I have to do 50 queries :S.</p></li>
  </ul>
  
  <p>Am I missing something here, is there a better way to do this?  I'm leaning towards blank root but it doesn't quite seem right</p>
  
  <p>Thanks. </p>