with users as  (
    select 1 as group_id, 1 as user_id, 10 as lvl
    union all
    select 1 as group_id, 2 as user_id, 8 as lvl
    union all
    select 1 as group_id, 3 as user_id, 0 as lvl

    union all
    select 2 as group_id, 2 as user_id, 100500 as lvl
    union all
    select 2 as group_id, 3 as user_id, 5 as lvl
),
 user_group as (
     select 1 as id, 10 priority
     union all
     select 2 as id, 1 priority
 ),
 tickets as (
     select 1 as id, 1 as group_id, 1 as position, null::int4 last_user_activity
     union all
     select 2 as id, 1 as group_id, 2 as position, null last_user_activity
     union all
     select 3 as id, 1 as group_id, 3 as position, null last_user_activity

     union all
     select 4 as id, 2 as group_id, 4 as position, 100500 last_user_activity
     union all
     select 5 as id, 2 as group_id, 5 as position, 3 last_user_activity
 )
select *
from (
    select tickets.id, users.user_id,
        row_number() over (
            partition by user_id
            order by
                user_group.priority desc,
                case when tickets.last_user_activity = users.user_id then 1000 * tickets.position else tickets.position end desc,
                tickets.position
            ) rn, *
    from tickets,
         users
        inner join user_group on user_group.id = users.group_id
    where tickets.group_id = users.group_id
) t
where t.rn = 1