Conjunctive queries determinacy and rewriting

• Authors: Daniel Pasailă
• Venue: Proceedings of the 14th International Conference on Database Theory
• Year: 2011

The problem of whether a query Q can be answered using a set of views V studies the possibility of computing Q when only the answers to the given set of views are available. In information-theoretic terms, we say that V determines Q iff for any two databases D1, D2, V(D1) = V(D2) implies Q(D1) = Q(D2). In the case that V determines Q, we also study the existence of equivalent rewritings of Q in terms of V in a specific rewriting language. Having a view language ν and a query language Q we say that ν-to-Q determinacy is decidable if there is an algorithm which, given a view V ε ν and a query Q ε Q, outputs whether V determines Q. We focus on the case where the views and the query are defined by subclasses of conjunctive queries and investigate in which cases V determines Q and the existence of equivalent rewritings of Q in terms of V. We define the class of CQcgraph queries as binary CQ queries whose body, if viewed as an undirected graph, is connected. Next, we establish necessary conditions for determinacy in the CQcgraph-to-CQcgraph case. We also show that CQchain-to-CQcgraph determinacy is decidable, extending the previous decidability result for CQchain-to-CQchain, where CQchain denotes the class of binary CQ queries whose body is a simple path between the two free variables. Finally, we provide an algorithm which, starting with a set of CQcgraph views V and an integer κ, generates a set of CQcgraph queries that are determined by V and have their size bounded by κ.