@Override
  protected HashTableLoader getHashTableLoader(Configuration hconf) {
    HashTableImplementationType hashTableImplementationType = vectorDesc.getHashTableImplementationType();
    HashTableLoader hashTableLoader;
    switch (vectorDesc.getHashTableImplementationType()) {
    case OPTIMIZED:
      // Use the Tez hash table loader.
      hashTableLoader = HashTableLoaderFactory.getLoader(hconf);
      break;
    case FAST:
      // Use our specialized hash table loader.
      hashTableLoader = new VectorMapJoinFastHashTableLoader();
      break;
    default:
      throw new RuntimeException("Unknown vector map join hash table implementation type " + hashTableImplementationType.name());
    }
    return hashTableLoader;
  }
This override lets us substitute our own fast vectorized hash table loader.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/mapjoin/VectorMapJoinCommonOperator.java