private T findInternal(T element) {
    Entry<T> value = map.get(element);
    if (value.getParent() == null) {
      return element;
    }
    T root = findInternal(value.getParent());
    value.setParent(root);
    return root;
  }
@return <tt>true</tt> if the specified equivalence is new
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/utils/DisjointSet.java