  We study privacy-preserving query answering in Description Logics (DLs).
Specifically, we consider the approach of controlled query evaluation (CQE)
based on the notion of instance indistinguishability. We derive data complexity
results for query answering over DL-Lite$_{\mathcal{R}}$ ontologies, through a
comparison with an alternative, existing confidentiality-preserving approach to
CQE. Finally, we identify a semantically well-founded notion of approximated
query answering for CQE, and prove that, for DL-Lite$_{\mathcal{R}}$
ontologies, this form of CQE is tractable with respect to data complexity and
is first-order rewritable, i.e., it is always reducible to the evaluation of a
first-order query over the data instance.
