EXTENDING CLASSICAL FUNCTIONAL-DEPENDENCIES FOR PHYSICAL DATABASE DESIGN

Traditionally, database design activities are partitioned into distinc t phases in which a logical design phase precedes physical database de sign. The objective of the logical design step is to eliminate redunda ncies and updating anomalies using the notion of data dependencies, wh ile leaving the physical design step to consider how the database sche ma may be restructured to provide more efficient access. We argue in t his paper that the separation of these two steps often results in phys ical database design not being able to benefit from knowledge of the s emantics of data captured in the earlier phases of the database design life cycle. As a step towards overcoming this problem, we demonstrate how classical functional dependencies can be extended to capture data semantics relevant to the design of database schemas which are more d esirable from the efficiency point of view. This is accomplished via t he introduction of strong and weak functional dependencies. Strong fun ctional dependencies indicate that the relationship between two attrib utes almost never change. This concept allows us to have controlled re dundancies which is beneficial as it can reduce dramatically the effor t needed to access frequently needed information. Weak functional depe ndencies capture the common situations in real life where classical fu nctional dependencies between two attributes hold in general but may b e violated in rare cases. Three new normal forms - the relaxed 3NF, re plicated 3NF and relax-replicated 3NF, induced by the strong and weak functional dependencies, provide a theoretical framework for designing database schemas which are more efficient and practical, while not co mpromising the integrity of the underlying database. That is, relation s in these new normal forms will not suffer from undesirable updating anomalies.