Restructuring of Relational Schemas

Schema restructuring is part of both database design and database reorganization, which are expressions of the specification and evolution of an information system. Entity-Relationship(ER) consistency expresses the capability of relational databases to model information oriented systems. A relational schema consistmg of relation schemes, together with key and inclusion dependencies, is said to be ER-consistent if it complies with an entity-relationship structure, meaning that it is representable by an Entity-Relationship diagram. We restrict our research of relational schema restructuring to ER-consistent schemas. The basic relational schelpa restructuring manipulations are the addition and removal of relation schemes, together with the adjustment of inner'and inter-relational dependencies. For relational schemas consisting of relation schemes, together with key and inclusion dependencies. we formally define the concept of incremental schema modification. Incrementality characterizes smooth schema transformations, without major disruptions. We define a set of ER-diagram transformations representing various information manipulations, and define their mapping to relational schema restructuring manipulations. We prove the correctness of the mapping, and show that the resulting schema restructuring manipulations are incremental. While incrementality characterizes one-step schema modifications, reversibility assures that every such modification can be undone also in one step; the proposed ER-diagram transformations are shown to be reversible. We define two types of comple(eness for the ER-diagram transformations, ERD-completeness and vertexcompleteness. A subset of basic ER-diagram transformations is shown to be ERD-'Complete, while the whole set of the ER-diagram transformations is shown to be venex-complete. Emai1[BI1NET]: {victor, janos}@TECHSEL T ec hn io n C om pu te r Sc ie nc e D ep ar tm en t T eh ni ca l R ep or t C S0 44 2 19 86