@Override
  public boolean equals(Object obj) {
    return (obj.getClass() == AuxOpTreeSignature.class) && sig.equals(((AuxOpTreeSignature) obj).sig);
  }
This signature is used to establish related parts to connect pre/post PPD optimizations.
ql/src/java/org/apache/hadoop/hive/ql/plan/mapper/AuxOpTreeSignature.java