code: func (cf *countFunction) GetType() *types.FieldType {
  	ft := types.NewFieldType(mysql.TypeLonglong)
  	ft.Flen = 21
  	ft.Charset = charset.CharsetBin
  	ft.Collate = charset.CollationBin
  	return ft
  }
docs: // GetType implements AggregationFunction interface.