@Override
  public ListenableFuture<?> isBlocked() {
    if (buildFinished) {
      return probeSource.isBlocked();
    }
    return buildSource.isBlocked();
  }
This Operator is used to implement the simple nested loop join algorithm for Cartesian product.
It is used to join two tables, one is the probe table and the other is the build table. For now,
the build table is assumed to be small enough to be cached in memory.(Produced by a scalar
subquery.) Scalar subquery is always the right child of PlanNode, so we can use right child of
JoinNode as the build table.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/process/join/SimpleNestedLoopCrossJoinOperator.java