code: public long getEstimateNumKeys(String cfName) throws RocksDbException {
          String prop = getProperty(cfName, "rocksdb.estimate-num-keys");
          return prop != null ? Long.parseLong(prop) : 0;
      }
docs: Gets estimated number of keys for a column family.
  
  @param cfName
  @return
  @throws RocksDbException