To add to this: every serious production system tends toward a slurry of 2NF-3NF-BCNF models, mostly for performance reasons. The case of lastMessageTime is a s

To add to this: every serious production system tends toward a slurry of 2NF-3NF-BCNF models, mostly for performance reasons. The case of lastMessageTime is a simpler one, as it might drift out of date but generally you get false positives but never a false negative. Note that people complain about Facebook message jewels showing up when there's nothing new but seldom complain about not getting notified. Another curious example is columnar databases. They are basically a 4NF storage layout with a veneer of 3NF syntax for ease of querying.