Decomposing relationship types by pivoting and schema equivalence

摘要

In the relational data model, the problem of data redundancy has been successfully tackled via decomposition. In advanced data models, decomposition by pivoting provides a similar concept. Pivoting has been introduced by Biskup, Menzel and Polle, and used for decomposing relationship types according to a unary non-key functional dependency. Our objective is to study pivoting in the presence of cardinality constraints which are commonly used in semantic data models. For this, we generalize the notion of pivoting such that decomposing relationship types does no longer require the existence of a given unary functional dependency. In order to ensure the equivalence of the given schema and its image under pivoting, the original application-dependent constraints have to be preserved. We discuss this problem for sets of participation and co-occurrence constraints. In particular, we prove the necessity of path cardinality constraints, and give an appropriate foundation for this concept.