[andrew_s1]

Think about scalability too – does it make sense to store a marker for every topic, given that it will grow with every new user and every new topic?

Another way to do it would be to store a “high tide mark” for every user on each forum, recording the highest post number that a user has read. That way, when they come back to the forum, you can do an easy SELECT on topics which have a post higher than the high tide mark, and flag those as having unread posts.