  The second Gaia data release is expected to contain data products from about
22 months of observation. Based on these data, we aim to provide an advance
publication of a full-sky Gaia map of RR Lyrae stars. Although comprehensive,
these data still contain a significant fraction of sources which are
insufficiently sampled for Fourier series decomposition of the periodic light
variations. The challenges in the identification of RR Lyrae candidates with
(much) fewer than 20 field-of-view transits are described. General
considerations of the results, their limitations, and interpretation are
presented together with prospects for improvement in subsequent Gaia data
releases.
