public void clear() {
    ttlCacheTree.removeAllChildren();
    ttlCacheTree.addChild(IoTDBConstant.MULTI_LEVEL_PATH_WILDCARD, Long.MAX_VALUE);
  }
@return key is path contains wildcard between each node
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/schema/ttl/TTLCache.java