The Denormalization Decision in Relational Database Systems Design - High-Performance Web Databases: Design, Development, and Deployment - page 670

Exhibit 49-2. Normalized relations to BCNF for WAREHOUSE_INFO.
• SQL query without using JOIN operation:
— SELECT social number, name, street, city, state, zip FROM
PERSON_INFO
• SQL query using JOIN operation:
— SELECT social number, name, street, city, state, zip FROM PERSON,
CODE WHERE PERSON. ZIP-CODE-ZIP
Normalizing the PERSON_INFO relation results in the PERSON and CODE
relations.
The JOIN operation is expensive. Relational operations on one relation
are intrinsically more efficient than the same operations on more than one
relation, even though the JOIN operation can be implemented efficiently
(e.g., the clustering of two files). Thus, normalization should be conducted
carefully, with consideration of costs involved.
A METHOD FOR MAKING A DENORMALIZATION DECISION FOR
TRANSITIVE FUNCTIONAL DEPENDENCY
In current practice of database systems design, a relational model is
designed and normalized up to a certain normal form (e.g., usually, Boyce-
Codd Normal Form or 4NF), and a normalized relation is subsequently
denormalized ad hoc, as necessary to improve performance. Unfortu-
nately, there are few guidelines for this denormalization.