  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.
