@Override
  public long getEstimatedMemorySize() {
    long size = 0;
    size += writeBuffers == null ? 0 : writeBuffers.getEstimatedMemorySize();
    return size;
  }
Two 64-bit long result is the key and value reference.
@param partialHashCode
@param keyBytes
@param keyStart
@param keyLength
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/mapjoin/fast/VectorMapJoinFastBytesHashSetStore.java