@Override
  public IDatabaseMNode<ICachedMNode> collectUpdatedStorageGroupMNodes() {
    IDatabaseMNode<ICachedMNode> storageGroupMNode = nodeBuffer.getUpdatedDatabaseMNode();
    nodeBuffer.removeUpdatedDatabaseNode();
    return storageGroupMNode;
  }
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