File definition and logical data independence

Abstract

This paper presents an architecture for a data base system which is capable of providing data independence at two levels, viz physical and logical. The architecture employs a schema, which gives an abstract picture of the physical data base, and a subschema, which contains the definitions of an application program's logical files. The language for writing these logical file definitions is then discussed, and a number of features which should be provided in such a language are identified, and described. In particular it is shown that the language should include the quantifiers of the predicate calculus, and should be capable of defining files which span several files in the schema. The use of the subschema in providing logical data independence is then demonstrated, and an appropriate application program discipline discussed. This is the first of two associated papers, the second of which deals with the complementary problems of storage structure and physical data independence.