@Override
    public boolean hasNext() {
      if (nextSubtree == null) {
        tryGetNext();
      }
      return nextSubtree != null;
    }
Collect updated storage group node.

@return null if not exist
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/pbtree/memory/MemoryManager.java