create type t_arand_num_state as (i int, r numeric);

create function arand_num_next (t_arand_num_state, numeric)
returns t_arand_num_state language sql as $$
select (
  $ 1 .i+ 1 ,
  case when  1 . 0 /$ 1 .i > random() then $ 2  else $ 1 .r end
)::t_arand_num_state
$$;

create function arand_num_final (t_arand_num_state)
returns numeric language sql as $$
select $ 1 .r
$$;

create aggregate arand (numeric) (
  stype     = t_arand_num_state,
  sfunc     = arand_num_next,
  finalfunc = arand_num_final,
  initcond  = '(1,)'
);


--test
select g, arand(n)
from (values( 1 , 1 ),( 1 , 2 ),( 1 , 3 ),( 1 , 4 ),( 1 , 5 ),( 1 , 6 ),( 2 , 10 ),( 2 , 11 ),( 2 , 12 )) s(g,n)
group by g