DouKaku?

=# create function lot(int, int) returns setof int as $$
  select * from generate_series(1, $1) order by random() limit $2;
$$ language sql;
=# select * from lot(10, 3);

普通は関数作るまでもなく、
=# select * from generate_series(1, 10) order by random() limit 3;