This is an old question. An alternative workaround is to use logs and exponentials, where product(y)=exp(sum(ln(y))). As SQL:

with
  d as (select 1 as x, 3 as y union all select 1 as x, 4 as y)
  select x, sum(y) as sum_y, exp(sum(ln(y))) as prod_y from d group by x;

1|7|12.0