private Object getKeyFor(Object o) {
    if (o instanceof Operator) {
      Operator<?> operator = (Operator<?>) o;
      return signatureCache.getSignature(operator);
    }
    return o;
  }
Links and optionally merges the groups identified by the two objects.
ql/src/java/org/apache/hadoop/hive/ql/plan/mapper/PlanMapper.java