title: Twitter-like "follow user" and "watch this" problem
body: <p>What's the best way to handle many-to-many trigger relationships like the Twitter "follow this user" problem.</p>
  
  <p>I have a similar problem with users "watching" threads for replies.  If there are 10,000 users watching a thread, and someone replies, how best to notify the watchers?  All I can think of is the following:</p>
  
  <p>Upon insert, check the "watch table" [fields: user_id, thread_id] for any thread matching this thread's id.  This is the list of users I need to notify.  For each user that needs to be notified, insert a row into the "notification table" [fields: user_id, message, addedon, etc].  Now I can show any user their notifications via this table.</p>
  
  <p>Problem is, this all sounds very, very expensive.  Especially the 10,000 inserts part.</p>
  
  <p>There must be a better way to do this...  ideas?</p>