public void firstBatchSetup(VectorizedRowBatch batch) throws HiveException {
    // Make sure small table BytesColumnVectors have room for string values in the big table and
    // overflow batchs...
    for (int column: smallTableByteColumnVectorColumns) {
      BytesColumnVector bytesColumnVector = (BytesColumnVector) batch.cols[column];
      bytesColumnVector.initBuffer();
    }

    // Setup a scratch batch that will be used to play back big table rows that were spilled
    // to disk for the Hybrid Grace hash partitioning.
    spillReplayBatch = VectorizedBatchUtil.makeLike(batch);
  }
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