Bounded query rewriting using views

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 D_Q 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 |D_Q| of D_Q and the time to identify D_Q 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 Σ^p₃-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.