Hydrolysis of starch by means of amylase to unclog bathtub drain
The problem you'd face using any chemical to dissolve the rice is that rice forms a thick gelatinous surface coating as it dissolves, so that it would take an extremely long time for the amylase to penetrate... perhaps months or years. You'd be better off using some of the standard mechanical drain-cleaning techniques:


Disassembly of the sink trap (and drain, if needed) and cleaning or replacing it
Drain auger
Drain snake
Water-pressure drain cleaner


Note that augers and snakes can scratch sinks or toilet bowls, and that water-pressure cleaners can cause pipes to come apart.

N.B. If you've already poured a potentially dangerous cleaner into the drain, remove it safely before trying anything else!