Managing Schema Versions in a Time-Versioned Non-First-Normal-Form Relational Database

Support of time versions is a very advanced feature in a DBMS. However, full flexibility of history processing is achieved only if we can also change the database schema dynamically, without touching the history. A technique for achieving this goal is here presented, in the frames of the Non-First- Normal-Form (NF2) relational data model. The environment is a pilot DBMS supporting this model, developed by the Advanced Information Management (AIM) project at the IBM Heidelberg Scientific Center. The technical solution pursues to minimize the storage space and the number of data versions. One way to achieve this is to avoid the immediate update of all data instances in the context of a schema change. Transformations between versions enable the correct interpretation of data. The management of time-related queries becomes complicated, when schema changes are involved. The paper describes a technique of applying global views over different schema versions, when formulating the queries and their results.