1. Выделение непрерывных групп
select
    sum(
        case
            when group_id = prev_group_id
            then 0
            else 1
        end
    ) as group_count
from (
    select
        group_id,
        lag(group_id) over () as prev_group_id
    from public.users
) as users

2. Количество записей в каждой группе
select
    group_id,
    count(*)
from (
    select
        *,
        row_number() over (order by id) - row_number() over (partition by group_id order by id) as group_seq
    from public.users
) as users
group by group_id, group_seq

3. Минимальный id записи в группе
select
    group_id,
    min(id)
from (
    select
        *,
        row_number() over (order by id) - row_number() over (partition by group_id order by id) as group_seq
    from public.user
) as users
group by group_id, group_seq