Rewritability in Monadic Disjunctive Datalog, MMSNP, and Expressive Description Logics.

We study rewritability of monadic disjunctive Datalog programs, (the complements of) MMSNP sentences, and ontology-mediated queries (OMQs) based on expressive description logics of the ALC family and on conjunctive queries. We show that rewritability into FO and into monadic Datalog (MDLog) are decidable, and that rewritability into Datalog is decidable when the original query satisfies a certain condition related to equality. We establish 2NEXPTIME-completeness for all studied problems except rewritability into MDLog for which there remains a gap between 2NEXPTIME and 3EXPTIME. We also analyze the shape of rewritings, which in the case of MMSNP correspond to obstructions, and give a new construction of canonical Datalog programs that is more elementary than existing ones and also applies to non-Boolean queries.