int size() {
      int size = 0;
      for (final Map<K, V> map : maps) {
        if (Objects.nonNull(map)) {
          size += map.size();
        }
      }
      return size;
    }
Since the capacity of one instance of ConcurrentHashMap is about 4 million, a number of
instances are united for more capacity.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/analyze/cache/schema/dualkeycache/impl/DualKeyCacheImpl.java