@Override
  public void close() throws Exception {
    if (schemaReader != null) {
      schemaReader.close();
      schemaReader = null;
    }
  }
Try to get next {@link TsBlock}. If the next is not ready, return a future. After success,
{@link SchemaQueryScanOperator#next} will be set.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/schema/SchemaQueryScanOperator.java