Map<K, V> getBelongedMap(final K key) {
      int slotIndex = key.hashCode() % SLOT_NUM;
      slotIndex = slotIndex < 0 ? slotIndex + SLOT_NUM : slotIndex;
      Map<K, V> map = maps[slotIndex];
      if (map == null) {
        synchronized (maps) {
          map = maps[slotIndex];
          if (map == null) {
            map = new ConcurrentHashMap<>();
            maps[slotIndex] = map;
          }
        }
      }
      return map;
    }
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