locked AS (
    SELECT o.id
      FROM outbox o
      JOIN candidate c ON c.id = o.id
     WHERE o.status = 'PENDING'                -- repeated on purpose
       AND o.available_at <= now()
       AND (o.claimed_at IS NULL OR o.claimed_at < now() - make_interval(secs => ?))
     ORDER BY o.id
     LIMIT ?
       FOR UPDATE OF o SKIP LOCKED
)