@Override public void readFields(DataInput in) throws IOException {
    super.readFields(in);
    druidQuery = in.readUTF();
    int length = in.readInt();
    String[] listHosts = new String[length];
    for (int i = 0; i < length; i++) {
      listHosts[i] = in.readUTF();
    }
    hosts = listHosts;
  }
Druid split. Its purpose is to trigger query execution in Druid.
druid-handler/src/java/org/apache/hadoop/hive/druid/io/HiveDruidSplit.java