@Override
  public PlanNodeId getSourceId() {
    return sourceId;
  }
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