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As noted in [STON80], versions (also called hypothetical relations) can be used to support "what if" scenarios in the database, and offer a mechanism for manipulating live data without fear of corrupting the database. In general, versions are implemented through differential files (deltas) [SEVR76], either via forward deltas [STON81, WOOD83], or via backward deltas [KATZ82]. The main drawback of version systems is that the semantics are desired. Furthermore, no single system is general enough to subsume all of the proposals. This thesis shows that production rules in a database system can be used to implement versions, and that such rules are general and flexible enough to accommodate any new semantics that the user might want.

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