title: how store many data like comments in database
body: <p>i have a catalog shopping system that has 9 major categories <br>
  it has over billion comments in each specific category<br>
  now i want redesign the site for optimizing performance<br>
  how could i store comments that has best performance when retrieving comments?<br>
  the comments are growing very fast, what about one year later that comments get bigger?<br>
  i think about separate comments for each specific category like comment_1, comment_2 and so on<br>
  what is the best design for storing comments?<br>
  and one question, how many rows does affect performance?<br></p>