From Conjunctive Queries to Instance Queries in Ontology-Mediated Querying

TL;DR

This paper investigates when ontology-mediated queries based on expressive description logics can be rewritten into simpler instance queries, providing characterizations, complexity bounds, and insights into query equivalence problems.

Contribution

It offers exact characterizations and tight complexity bounds for the rewritability of ontology-mediated queries into instance queries, advancing understanding of query optimization in description logics.

Findings

Exact characterizations of when rewritability is possible

Tight complexity bounds for deciding rewritability

Complexity analysis of MMSNP sentence equivalence to CSP

Abstract

We consider ontology-mediated queries (OMQs) based on expressive description logics of the ALC family and (unions) of conjunctive queries, studying the rewritability into OMQs based on instance queries (IQs). Our results include exact characterizations of when such a rewriting is possible and tight complexity bounds for deciding rewritability. We also give a tight complexity bound for the related problem of deciding whether a given MMSNP sentence is equivalent to a CSP.