  We investigate potential gains in cosmological constraints from the
combination of galaxy clustering and galaxy-galaxy lensing by optimizing the
lens galaxy sample selection using information from Dark Energy Survey (DES)
Year 3 data and assuming the DES Year 1 Metacalibration sample for the sources.
We explore easily reproducible selections based on magnitude cuts in $i$-band
as a function of (photometric) redshift, $z_{\rm phot}$, and benchmark the
potential gains against those using the well established redMaGiC sample. We
focus on the balance between density and photometric redshift accuracy, while
marginalizing over a realistic set of cosmological and systematic parameters.
Our optimal selection, the MagLim sample, satisfies $i < 4 \, z_{\rm phot} +
18$ and has $\sim 30\%$ wider redshift distributions but $\sim 3.5$ times more
galaxies than redMaGiC. Assuming a wCDM model and equivalent scale cuts to
mitigate nonlinear effects, this leads to $40\%$ increase in the figure of
merit for the pair combinations of $\Omega_m$, $w$, and $\sigma_8$, and gains
of $16\%$ in $\sigma_8$, $10\%$ in $\Omega_m$, and $12\%$ in $w$. Similarly, in
LCDM we find an improvement of $19\%$ and $27\%$ on $\sigma_8$ and $\Omega_m$,
respectively. We also explore flux-limited samples with a flat magnitude cut
finding that the optimal selection, $i < 22.2$, has $\sim 7$ times more
galaxies and $\sim 20\%$ wider redshift distributions compared to MagLim, but
slightly worse constraints. We show that our results are robust with respect to
the assumed galaxy bias and photometric redshift uncertainties with only
moderate further gains from increased number of tomographic bins or the
inclusion of bin cross-correlations, except in the case of the flux-limited
sample, for which these gains are more significant.
