Reconcilable differences
Todd J. Green, Zachary G. Ives, Val Tannen · 2009
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 (updating 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. We present a novel way of managing this difficulty via an excursion through a non-standard semantics, Z-relations, where tuples are annotated with positive or negative integers.