V get(final K key) {
      return getBelongedMap(key).get(key);
    }
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