gistfile1.txt

drop table if exists user_course cascade;
drop table if exists users cascade;
drop table if exists courses cascade;

create table users(id serial,data text, primary key(id));
insert into users(data) select lpad('',100,md5(id::text))
from generate_series(1,1000000) t(id);

create table courses(id serial,data text,primary key(id));
insert into courses(data) select lpad('',100,md5(id::text))
from generate_series(1,100000) t(id);

create table user_course(course_id bigint, user_id bigint, primary key(user_id,course_id), 
foreign key(course_id) references courses(id),
foreign key(user_id) references users(id));

do $$
declare c record;
begin
 for c in select * from courses loop
	for i in 1..10 loop
	  begin
	 insert into user_course(course_id,user_id) values(
		c.id, 
		(random()*1000000)::bigint
		);
	   exception when unique_violation then
	   end;
	end loop;
 end loop;
end
$$;

analyze;

create or replace function unassigned_users_top_10(bigint) returns setof users as '
select * from users
where not exists
(select user_id from user_course 
where course_id = $1 and 
users.id = user_id) order by id limit 10;
' language sql;

do $$
declare i bigint;
declare rnd bigint;
begin
	for i in  select a from generate_series(1,1000) t(a) loop
		perform unassigned_users_top_10((i * 13*19*17 % 100000)::bigint);
	end loop;
end
$$;