Postgresql:Genera sequenza

Qualcosa del genere?

select x, string_agg((array['A', 'C', 'G', 'T'])[1 + floor(random() * 4)], '')
from generate_series(1, 20, 1) gsn(n) cross join
     generate_series(1, 10, 1) gsx(x)
group by x