The Cost of Normalization
There can be a cost to normalization when the logical model is implemented as a 1‑to‑1 physical mapping. In a very real sense, normalization optimizes update performance at the expense of retrieval performance. Because of this, it sometimes becomes necessary to “denormalize” physical tables to some extent in order to achieve reasonable overall system performance. This so‑called denormalization is actually an implementation issue, not a logical design issue (see Chapter 6: “Denormalizing the Physical Schema,” for a description of “denormalization.”)
Before examining the details of the normalization process, a number of properties of relations and their manipulation need to be spelled out.