code: func needDumpStatsDelta(h *Handle, id int64, item variable.TableDelta, currentTime time.Time) bool {
  	if item.InitTime.IsZero() {
  		item.InitTime = currentTime
  	}
  	tbl, ok := h.StatsCache.Load().(StatsCache)[id]
  	if !ok {
  		// No need to dump if the stats is invalid.
  		return false
  	}
  	if currentTime.Sub(item.InitTime) > dumpStatsMaxDuration {
  		// Dump the stats to kv at least once an hour.
  		return true
  	}
  	if tbl.Count == 0 || float64(item.Count)/float64(tbl.Count) > DumpStatsDeltaRatio {
  		// Dump the stats when there are many modifications.
  		return true
  	}
  	return false
  }
docs: // needDumpStatsDelta returns true when only updates a small portion of the table and the time since last update
  // do not exceed one hour.