View-based query processing: on the relationship between rewriting, answering and losslessness

• Authors: Diego Calvanese;Giuseppe De Giacomo;Maurizio Lenzerini;Moshe Y. Vardi
• Venue: ICDT'05 Proceedings of the 10th international conference on Database Theory
• Year: 2005

As a result of the extensive research in view-based query processing, three notions have been identi.ed as fundamental, namely rewriting, answering, and losslessness. Answering amounts to computing the tuples satisfying the query in all databases consistent with the views. Rewriting consists in first reformulating the query in terms of the views and then evaluating the rewriting over the view extensions. Losslessness holds if we can answer the query by solely relying on the content of the views. While the mutual relationship between these three notions is easy to identify in the case of conjunctive queries, the terrain of notions gets considerably more complicated going beyond such a query class. In this paper, we revisit the notions of answering, rewriting, and losslessness and clarify their relationship in the setting of semistructured databases, and in particular for the basic query class in this setting, i.e., two-way regular path queries. Our .rst result is a clean explanation of the relationship between answering and rewriting, in which we characterize rewriting as a “linear approximations” of query answering. We show that applying this linear approximation to the constraint-satisfaction framework yields an elegant automata-theoretic approach to query rewriting. As for losslessness, we show that there are indeed two distinct interpretations for this