alter table public.users
  add column if not exists username text null,
  add column if not exists password_hash text null;

do $$
begin
  if not exists (
    select 1
    from pg_indexes
    where schemaname = 'public'
      and indexname = 'users_username_unique'
  ) then
    create unique index users_username_unique on public.users (username) where username is not null;
  end if;
end $$;