Bounded Query Rewriting Using Views

A query Q in a language L has a bounded rewriting using a set of L-definable views if there exists a query Q′ in L such that given any 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, which are a combination of simple 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 L, from Σp3-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. Finally, we investigate L1-to-L2 bounded rewriting, when Q in L1 is allowed to be rewritten into a query Q′ in another language L2. We show that this relaxation does not simplify the analysis of bounded query rewriting using views.