@Override
  public void startGroup() throws HiveException {

    // We do not call startGroup on operators below because we are batching rows in
    // an output batch and the semantics will not work.
    // super.startGroup();
    throw new HiveException("Unexpected startGroup");
  }
changes the processing mode to streaming
This is done at the request of the hash agg mode, if the number of keys
exceeds the minReductionHashAggr factor
@throws HiveException
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/VectorGroupByOperator.java