Recent experiments have shown that the perovskite to post-perovskite phase transformation in CaIrO_{3} occurs more readily at room temperature when a shear stress is applied as compared to isotropic pressure. To understand this mechanistically, we have calculated the minimum-energy pathway of the phase transition with density functional theory under different pressure conditions with the generalized solid-state nudged elastic band method. Our results reveal that shear stress significantly lowers the barrier and stabilizes the product state while isotropic pressure initially raises the barrier and only reduces the barrier at pressures above 90 GPa. The nonmonotonic change in barrier with isotropic pressure is explained in terms of an increase in the activation volume under low pressure and a decrease under high pressure.