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Exact query reformulation using views in positive relational languages is well understood, and has a variety of applications in query optimization and data sharing. Generalizations to larger fragments of the Relational Algebra (RA)- specifically, support for the difference operator - would increase the options available for query reformulation, and also apply to view adaptation (up-dating a materialized view in response to a modified view definition) and view maintenance. Unfortunately, most questions about queries become undecidable in the presence of difference/negation.
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