@Override
    public byte updateStateByte(Byte previousValue) {
      if (!hasFilterTag) return (byte)0xff;
      byte aliasFilter = (previousValue == null) ? (byte)0xff : previousValue.byteValue();
      aliasFilter &= ((ShortWritable)valObjs[valObjs.length - 1]).get();
      return aliasFilter;
    }
Get hash value from the key.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/MapJoinBytesTableContainer.java