void clear() {
      synchronized (maps) {
        Arrays.fill(maps, null);
      }
    }
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