code: func (hg *Histogram) greaterRowCount(value types.Datum) float64 {
  	gtCount := hg.notNullCount() - hg.lessRowCount(value) - hg.equalRowCount(value)
  	if gtCount < 0 {
  		gtCount = 0
  	}
  	return gtCount
  }
docs: // greaterRowCount estimates the row count where the column greater than value.