  We study phase transitions and hysteresis in a system of dipolar bosons
loaded into triangular optical lattices at zero temperature. We find that the
quantum melting transition from supersolid to superfluid phase is first-order,
in contrast with the previous report. We also find that due to strong quantum
fluctuations the supersolid (or solid)-superfluid transition can exhibit an
anomalous hysteretic behavior, in which the curve of density versus chemical
potential does not form a standard loop structure. Furthermore, we show that
the transition occurs unidirectionally along the anomalous hysteresis curve.
