Storage structure and physical data independence

This paper presents the results of an investigation into the feasibility of physical data independence in a data base system. In theory physical data independence may be achieved by providing a picture of the data which remains invariant as the underlying storage structure of the data base changes. In this paper the data picture is assumed to describe the data base as consisting of a set of records in third normal form. The question is, given this third normal form data picture, to what extent may the storage structure change? The paper attempts to answer this question by first defining and explaining a number of concepts, including the important notions of storage equivalence and conformable representation, and then using these concepts to illustrate some possible storage structures for a sample data base. It is demonstrated in particular that the number of possible conformable representations of the data base is potentially very large, and that certain advantages accrue from the choice of a conformable representation. Some non-conformable representations are also shown. This is the second of two associated papers, the first of which deals with the complementary problems of file definition and logical data independence.