code: func (af *avgFunction) GetType() *types.FieldType {
  	ft := types.NewFieldType(mysql.TypeNewDecimal)
  	ft.Charset = charset.CharsetBin
  	ft.Collate = charset.CollationBin
  	ft.Decimal = af.Args[0].GetType().Decimal
  	return ft
  }
docs: // GetType implements AggregationFunction interface.