Fundamental Concepts of Object Oriented Databases

It is claimed that object oriented databases (OODBs) overcome many of the limitations of the relational model. However, the formal foundation of OODB concepts is still an open problem. Even worse, for relational databases a commonly accepted datamodel existed very early on whereas for OODBs the uniication of concepts is missing. The work reported in this paper contains the results of our rst investigations on a formally founded object oriented datamodel (OODM) and is intended to contribute to the development of a uniform mathematical theory of OODBs. A clear distinction between objects and values turns out to be essential in the OODM. Types and Classes are used to structure values and objects repectively. Then the problem of unique object identiication occurs. We show that this problem can be be solved for classes with extents that are completely representable by values. Such classes are called value-representable. Another advantage of the relational approach is the existence of structurally determined generic update operations. We show that this property can be carried over to object-oriented datamodels if classes are value-representable. Moreover, in this case database consistency with respect to implicitly speciied referential and inclusion constraints will be automatically preserved. This result can be generalized with respect to distinguished classes of explicitly stated static constraints. Given some arbitrary method and some integrity constraint there exists a greatest consistent specialization (GCS) that behaves nice in that it is compatible with the conjunction of constraints. We present an algorithm for the GCS construction of user-deened methods and describe the GCSs of generic update operations that are required herein.