-- Sanitise the username, and make it unique if necessary.
...
  select (
    case
    when i = 0 then v_username
    else v_username || i::text
    end
  ) into v_username from generate_series(0, 1000) i
  where not exists(
    select 1
    from app_public.users
    where users.username = (
      case
      when i = 0 then v_username
      else v_username || i::text
      end
    )
  )
  limit 1;

  select (
    case
    when i = 0 then v_username
    else v_username || i::text
    end
  ) into v_username from generate_series(0, 1000) i
  where not exists(
    select 1
    from app_public.users
    -- comparing using lowercase to make sure the username is unique (username column is citext -> constraint is not case sensitive)
    where lower(users.username) = (
      case
      when i = 0 then lower(v_username)
      else lower(v_username) || i::text
      end
    )
  )
  limit 1;