June 1, 1993 Page 1

Derived Types and Subschemas:

Towards Better Support for Logical Data

Independence in Object-Oriented Data Models

Andreas Geppert
Stefan Scherrer
Klaus R. Dittrich

Technical Report 93.27
Institut f?r Informatik, Universit?t Z?rich1

Abstract

Two concepts for the support of logical data independence in the context of the structurally object-oriented data model NO2 (New Object-Oriented data model) are presented: derived types and subschemas. In general, object types can be derived in two ways: derived types can restrict (the value set of) an existing base type, or new atrtibutes can be added to the tuple value set of an existing object type. One major design issue of the concept is to preserve object identity: an instance of a derived type has the same object identity as the corresponding instance of the base type. This enables us to permit updates to derived types even in cases where (say) the relational view mechanism could not permit them. The second concept, subschemas, supports application-specific views of schemas and the straightforward identification of those parts affected by schema modifications.

1 Introduction and Motivation

NO2 is the data model of the structurally object-oriented database system CoOMS (Combined Object Management System; [Dumm 91]) which is currently under implementation at SNI2. NO2 offers a powerful data definition language (DDL) for the modelling of objects and relationships between them.

Nevertheless, an application may desire to view objects according to a structure different from the one defined in the underlying schema. More precisely,

? a user (in the sequel, let ?user? include application programs as well) may not be interested in (parts of) the values of objects. In the university example (fig. 1; explained in more detail in section 2), for instance, a user may desire to have an overview of departments, but not of the concrete research groups and their assistants.

1. Authors? address: Institut fuer Informatik, Universitaet Zuerich, Winterthurerstr. 190, CH-8057 Zuerich, Switzerland. Email: {geppert,scherrer,dittrich}@ifi.unizh.ch
2. Siemens-Nixdorf Informationssysteme