Overnormalization

Question

When would a database design be described as overnormalized? Is this characterization an absolute one? Or is it dependent on the way it is used in the application? Thanks.

Answer 1

It's always a question of the application domain. It's usually a question of correctness, but occasionally a question of performance.

There's one case where I can think of a prima facie case of overnormalization: say you have an order + orderitem, and the orderitem references productID, and leaves pricing to the product.price. Since that introduces temporal coupling, you've incorrectly normalized because the overnormalization affects already shipped orders, unless prices absolutely never change. You can certainly argue that this is simply a modeling error (as in the comments), but I see under-normalization as a modeling error in most cases, too.

The other category is performance related. In principle, I think there are generally better solutions to performance than denormalizing data, such as materialized views, but if your application suffers from the performance consequences of many joins, it may be worth assessing whether denormalizing can help you. I think these cases are often over-emphasized, because people sometimes reach for denormalization before they properly profile their application.

People also often forget about alternatives, like keeping a canonical form of the database and using warehousing or other strategies for frequently-read, but infrequently changed data.