docs: public void addScannerStat(final int query_index, final int id, 
        final QueryStat name, final long value) {
      Map<Integer, Map<QueryStat, Long>> qs = scanner_stats.get(query_index);
      if (qs == null) {
        qs = new ConcurrentHashMap<Integer, Map<QueryStat, Long>>(
                Const.SALT_WIDTH() > 0 ? Const.SALT_BUCKETS() : 1);
        scanner_stats.put(query_index, qs);
      }
      Map<QueryStat, Long> scanner_stat_map = qs.get(id);
      if (scanner_stat_map == null) {
        scanner_stat_map = new HashMap<QueryStat, Long>();
        qs.put(id, scanner_stat_map);
      }
      scanner_stat_map.put(name, value);
    }