A tetrachotomy of ontology-mediated queries with a covering axiom

Abstract

Aiming to understand the data complexity of answering conjunctive queries mediated by an axiom stating that a class is covered by the union of two other classes, we show that deciding their first-order rewritability is PSPACE-hard and obtain a number of sufficient conditions for membership in AC 0 , L, NL, and P. Our main result is a complete syntactic AC 0 /NL/P/CONP tetrachotomy of path queries under the assumption that the covering classes are disjoint.