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We provide an ultimately fine-grained analysis of the data complexity andrewritability of ontology-mediated queries (OMQs) based on an EL ontology and aconjunctive query (CQ). Our main results are that every such OMQ is in AC0,NL-complete, or PTime-complete and that containment in NL coincides withrewritability into linear Datalog (whereas containment in AC0 coincides withrewritability into first-order logic). We establish natural characterizationsof the three cases in terms of bounded depth and (un)bounded pathwidth, andshow that every of the associated meta problems such as deciding wether a givenOMQ is rewritable into linear Datalog is ExpTime-complete. We also give a wayto construct linear Datalog rewritings when they exist and prove that there isno constant Datalog rewritings.

A Complete Classification of the Complexity and Rewritability of Ontology-Mediated Queries based on the Description Logic EL