code: func (sf *sumFunction) CalculateDefaultValue(schema Schema, ctx context.Context) (d types.Datum, valid bool) {
  	arg := sf.Args[0]
  	result, err := EvaluateExprWithNull(ctx, schema, arg)
  	if err != nil {
  		log.Printf("Evaluate expr with null failed in function %s, err msg is %s", sf, err.Error())
  		return d, false
  	}
  	if con, ok := result.(*Constant); ok {
  		d, err = calculateSum(ctx.GetSessionVars().StmtCtx, d, con.Value)
  		if err != nil {
  			log.Printf("CalculateSum failed in function %s, err msg is %s", sf, err.Error())
  		}
  		return d, err == nil
  	}
  	return d, false
  }
docs: // CalculateDefaultValue implements AggregationFunction interface.