@Override
  protected void releaseResource() {
    driverContext.getFragmentInstanceContext().decrementNumOfUnClosedDriver();
  }
One {@link SchemaDriver} is used to execute one {@link FragmentInstance} which is for metadata
query.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/driver/SchemaDriver.java