  We show that, for OWL 2 QL ontology-mediated queries with (i) ontologies of
bounded depth and conjunctive queries of bounded treewidth, (ii) ontologies of
bounded depth and bounded-leaf tree-shaped conjunctive queries, and (iii)
arbitrary ontologies and bounded-leaf tree-shaped conjunctive queries, one can
construct and evaluate nonrecursive datalog rewritings by, respectively,
LOGCFL, NL and LOGCFL algorithms, which matches the optimal combined
complexity.
