Bounded Query Rewriting Using Views

Abstract

A query Q has a bounded rewriting using a set of views if there exists a query Q' expressed in the same language as Q, such that given a dataset D, Q(D) can be computed by Q' that accesses only cached views and a small fraction DQ of D. We consider datasets D that satisfy a set of access constraints, a combination of cardinality constraints and associated indices, such that the size |DQ| of DQ and the time to identify DQ are independent of |D|, no matter how big D is. This paper studies the problem for deciding whether a query has a bounded rewriting given a set V of views and a set A of access constraints. We establish the complexity of the problem for various query languages, from Σ3p-complete for conjunctive queries (CQ), to undecidable for relational algebra (FO). We show that the intractability for CQ is rather robust even for acyclic CQ with fixed V and A, and characterize when the problem is in PTIME. To make practical use of bounded rewriting, we provide an effective syntax for FO queries that have a bounded rewriting. The syntax characterizes a core subclass of such queries without sacrificing the expressive power, and can be checked in PTIME.