Versioned Relations: Support for Conditional Schema Changes and Schema Versioning

Abstract

We introduce the versioned relational data model, which allows a user to apply conditional schema changes to a populated database without breaking applications compiled against an existing schema, and without loss of existing data. Our model is based on keeping a history of conditional schema changes, and converting tuples on demand to fit the correct schema in any schema version.

We provide a concrete definition of schema versioning: The ability to specify an operator on any schema version, such that the tuples in the result are unaffected by schema versions created after the specified schema version. Finally, we show that our model supports schema versioning.