code: func getSingletonStats(len int) *property.StatsInfo {
  	ret := &property.StatsInfo{
  		RowCount:    1.0,
  		Cardinality: make([]float64, len),
  	}
  	for i := 0; i < len; i++ {
  		ret.Cardinality[i] = 1
  	}
  	return ret
  }
docs: // Exists and MaxOneRow produce at most one row, so we set the RowCount of stats one.