code: func ExplainAggFunc(agg *AggFuncDesc) string {
  	var buffer bytes.Buffer
  	fmt.Fprintf(&buffer, "%s(", agg.Name)
  	if agg.HasDistinct {
  		buffer.WriteString("distinct ")
  	}
  	for i, arg := range agg.Args {
  		buffer.WriteString(arg.ExplainInfo())
  		if i+1 < len(agg.Args) {
  			buffer.WriteString(", ")
  		}
  	}
  	buffer.WriteString(")")
  	return buffer.String()
  }
docs: // ExplainAggFunc generates explain information for a aggregation function.