title: new comments notification implementation for a web site
body: <p>I've got a website where people can make posts and comment on them. I want to make a new comment notification system, so when a user logs in he can see number of new comments to his posts.</p>
  
  <p>My idea is to add a 'read' table, where I will store user_id and comment_id (means user_id read comment_id). But this can make some performance issue, when table will grow big.</p>
  
  <p>What's the best way to implement this?</p>