List<K> getAllKeys() {
      final List<K> res = new ArrayList<>();
      Arrays.stream(maps)
          .iterator()
          .forEachRemaining(
              map -> {
                if (map != null) {
                  res.addAll(map.keySet());
                }
              });
      return res;
    }
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