public AuxOpTreeSignature getAuxSignatureOf(Operator<?> op) {
    OpTreeSignature x = auxSignatureCache.getSignature(op);
    return new AuxOpTreeSignature(x);
  }
Links and optionally merges the groups identified by the two objects.
ql/src/java/org/apache/hadoop/hive/ql/plan/mapper/PlanMapper.java