Bounded Query Rewriting Using Views

Abstract

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 D Q 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 | D Q | of D Q and the time to identify D Q are independent of | D |, no matter how big D is. In this article, we study 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 Σ 3 p -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 key subclass of such queries without sacrificing the expressive power, and can be checked in PTIME. Finally, we investigate L 1 -to- L 2 bounded rewriting, when Q in L 1 is allowed to be rewritten into a query Q ′ in another language L 2 . We show that this relaxation does not simplify the analysis of bounded query rewriting using views.