I will do microblogging web service (for school, so don't blast me for lack of new idea) and I worry that DB could be often be overloaded (user could following other users or even tag so I suppouse that SELECT will be heavy - check 20 latest messages which contains all observing tags and user).
My idea is create another table, and store in it only statusID and userID (who should pick up message). Danger of that is, if some tag or user has many followers there will be a lot of record with that status ID. So, is it good idea? Or maybe better is used M2M relation? (one status -> many receivers)
I think most databases can easily handle large record sets. The responsibility to have it preform lies in your design with properly setting up the indexes. If you create the right indexes the select clauses should perform really well.