  The shear measurement from DECaLS (Dark Energy Camera Legacy Survey) provides
an excellent opportunity for galaxy-galaxy lensing study with DESI (Dark Energy
Spectroscopic Instrument) galaxies, given the large ($\sim 9000$ deg$^2$) sky
overlap. We explore this potential by combining the DESI 1\% survey and DECaLS
DR8. With $\sim 106$ deg$^2$ sky overlap, we achieve significant detection of
galaxy-galaxy lensing for BGS and LRG as lenses. Scaled to the full BGS sample,
we expect the statistical errors to improve from $18(12)\%$ to a promising
level of $2(1.3)\%$ at $\theta>8^{'}(<8^{'})$. This brings stronger
requirements for future systematics control. To fully realize such potential,
we need to control the residual multiplicative shear bias $|m|<0.01$ and the
bias in the mean redshift $|\Delta z|<0.015$. We also expect significant
detection of galaxy-galaxy lensing with DESI LRG/ELG full samples as lenses,
and cosmic magnification of ELG through cross-correlation with low-redshift
DECaLS shear. {If such systematical error control can be achieved,} we find the
advantages of DECaLS, comparing with KiDS (Kilo Degree Survey) and HSC
(Hyper-Suprime Cam), are at low redshift, large-scale, and in measuring the
shear-ratio (to $\sigma_R\sim 0.04$) and cosmic magnification.
