public void clear() {
    store.clear();
    storageGroupMNode = null;
  }
The hierarchical struct of the Metadata Tree is implemented in this class.

<p>Since there are too many interfaces and methods in this class, we use code region to help
manage code. The code region starts with //region and ends with //endregion. When using Intellij
Idea to develop, it's easy to fold the code region and see code region overview by collapsing
all.

<p>The codes are divided into the following code regions:

<ol>
  <li>MTree initialization, clear and serialization
  <li>Timeseries operation, including create and delete
  <li>Entity/Device operation
  <li>Interfaces and Implementation for metadata info Query
  <li>Interfaces and Implementation for MNode Query
  <li>Interfaces and Implementation for Template check
</ol>
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/mem/MTreeBelowSGMemoryImpl.java