code: func (c *Column) AvgColSize(count int64) float64 {
  	if count == 0 {
  		return 0
  	}
  	switch c.Histogram.Tp.Tp {
  	case mysql.TypeFloat:
  		return 4
  	case mysql.TypeTiny, mysql.TypeShort, mysql.TypeInt24, mysql.TypeLong, mysql.TypeLonglong,
  		mysql.TypeDouble, mysql.TypeYear:
  		return 8
  	case mysql.TypeDuration, mysql.TypeDate, mysql.TypeDatetime, mysql.TypeTimestamp:
  		return 16
  	case mysql.TypeNewDecimal:
  		return types.MyDecimalStructSize
  	default:
  		// Keep two decimal place.
  		return math.Round(float64(c.TotColSize)/float64(count)*100) / 100
  	}
  }
docs: // AvgColSize is the average column size of the histogram.