Database schema evolution through the specification and maintenance of changes on entities and relationships

Abstract

A flexible database system needs to support changes to its schema in order to facilitate the requirements of new applications and to support interoperability within a multidatabase system. In this paper, we present an approach to schema evolution through changes to the Entity-Relationship (ER) schema of a database. We enhance the graphical constructs used in ER diagrams, and develop EVER, an EVolutionary ER diagram for specifying the derivation relationships between schema versions, relationships among attributes, and the conditions for maintaining consistent views of programs. Algorithms are presented for mapping the EVER diagram into the underlying database and constructing database views for schema versions. Through the reconstruction of views after database reorganization, changes to an ER diagram can be made transparent to the application programs while all objects in the database remain accessible to the application programs.