Mourad Bouneffa, Nacer Boudjlida. Managing Schema Changes in Object-Relationship Databases. In Mike P. Papazoglou, editor, OOER 95: Object-Oriented and Entity-Relationship Modelling, 14th International Conference, Gold Coast, Australia, December 12-15, 1995, Proceedings. Volume 1021 of Lecture Notes in Computer Science, pages 113-122, Springer, 1995. [doi]
Changing a database schema may affect existing data and application programs. Immediate data restructuring is a first usual strategy that transforms the existing database to render it conform to the modified schema, but it does not deal with program adaptation to the changes. An other approach allows the existence of several schema versions for a single database: emulation mechanisms are then used to achieve program adaptation but they cannot avoid data redundancy. Furthermore, both the approaches only considered atomic schema changes. This paper proposes a combination of both the approaches: several facets of the database are virtually maintained: every facet represents the database in conformance to a schema version and only one reference facet may be physically stored. Mapping functions among the facets are also maintained: they deliver data items (to a user or to an application program) according to their definitions in a given version of the database schema.