Supporting multiple user views of databases is currently an important problem area in database management system development. An interesting facet of this problem arises whenever a user needs an integrated view of several distinct databases. Using traditional database concepts, an aggregate schema facility has been developed to address this problem. The basic functions of an aggregate schema facility are discussed, as well as their implementation in a CODASYL/DBTG-like environment. Interest in an aggregate schema facility grew out of a problem in restructuring large databases. The application of this facility to restructuring is discussed, as well as potential applications to dynamic translation and distributed databases.

The development of a powerful restructuring function involves two important components—the unambiguous specification of the restructuring operations and the realization of these operations in a software system. This paper is directed to the first component in the belief that a precise specification will provide a firm foundation for the development of restructuring algorithms and, subsequently, their implementation. The paper completely defines the semantics of the restructuring of tree structured databases. The delineation of the restructuring function is accomplished by formulating three different levels of abstraction, with each level of abstraction representing successively more detailed semantics of the function. At the first level of abstraction, the schema modification, three types are identified—naming, combining, and relating; these three types are further divided into eight schema operations. The second level of abstraction, the instance operations, constitutes the transformations on the data instances; they are divided into group operations such as replication, factoring, union, etc., and group relation operations such as collapsing, refinement, fusion, etc. The final level, the item value operations, includes the actual item operations, such as copy value, delete value, or create a null value.

The development of a powerful restructuring function involves two important components - the unambiguous specification of the restructuring operations and the realization of these operations in a software system. We direct our efforts to the first component in the belief that a precise specification will provide a firm foundation for restructuring algorithms and implementations. This paper defines completely the semantics of the restructuring of tree structured data bases.