Ontology-Based Query Answering over Datalog-Expressible Rule Sets is Undecidable

Ontology-based query answering is a problem that takes as input a set of facts F, an ontology R (typically expressed by existential rules), a Boolean query q , and asks whether R and F entails q. This problem is undecidable in general, and a widely investigated approach to tackle it is called query rewriting: from (R,q) (a ``rule query'') is computed q_R such that for any set of facts F, it holds that R and F entail q iff F entails q_R. The literature mostly focused on q_R expressed as a union of conjunctive queries (UCQs), and an algorithm that such a q_R whenever it exists has been proposed in the literature. However, UCQ-rewritability is applicable only in restricted settings. This raises the question whether such a generic algorithm can be designed for a more expressive language, such as datalog. We solve this question by the negative, by studying the difference between datalog-expressibility and datalog-rewritability. In particular, we show that query answering under datalog-expressible rule queries is undecidable.