drop function if exists stat();
create function stat() returns table(min_id bigint, group_id bigint, counter bigint) as $$
declare
	item users;
	prev_group_id bigint;
begin
	counter := 0;
	for item.id, item.group_id in select users.id, users.group_id from users order by users.id asc
	loop
		if(prev_group_id != item.group_id and prev_group_id is not null ) then
			return next;
			counter := 0;
			min_id := null;
		end if;

		group_id := item.group_id;
		counter := counter + 1;
		if(item.id < min_id or min_id is null) then
			min_id := item.id;
		end if;
		prev_group_id := item.group_id;
	end loop;
	return next;
end


$$ language plpgsql;


-- использование

select * from stat();