private boolean ensureBatch() throws IOException {
    if (rowInBatch >= batch.size) {
      rowInBatch = 0;
      return vrbReader.next(key, batch) && batch.size > 0;
    }
    return true;
  }
If the current batch is empty, get a new one.
@return true if we have rows available.
ql/src/java/org/apache/hadoop/hive/ql/io/BatchToRowReader.java