Rewriting Ontological Queries into Small Nonrecursive Datalog Programs

TL;DR

This paper demonstrates that ontological queries in DL-Lite and Linear Datalog+/- can be rewritten into polynomial-sized nonrecursive Datalog programs, improving efficiency over previous exponential-size methods.

Contribution

It introduces a method to produce polynomial-sized nonrecursive Datalog rewritings for certain ontological query problems, reducing complexity.

Findings

Rewritings can be polynomial in size, not exponential.

The method applies to DL-Lite and Linear Datalog+/- with role inclusion.

Efficient query evaluation is enabled by the new rewriting approach.

Abstract

We consider the setting of ontological database access, where an Abox is given in form of a relational database D and where a Boolean conjunctive query q has to be evaluated against D modulo a Tbox T formulated in DL-Lite or Linear Datalog+/-. It is well-known that (T,q) can be rewritten into an equivalent nonrecursive Datalog program P that can be directly evaluated over D. However, for Linear Datalog? or for DL-Lite versions that allow for role inclusion, the rewriting methods described so far result in a nonrecursive Datalog program P of size exponential in the joint size of T and q. This gives rise to the interesting question of whether such a rewriting necessarily needs to be of exponential size. In this paper we show that it is actually possible to translate (T,q) into a polynomially sized equivalent nonrecursive Datalog program P.