code: func (cf *concatFunction) GetResult(evalCtx *AggEvaluateContext) (d types.Datum) {
  	if evalCtx.Buffer != nil {
  		d.SetString(evalCtx.Buffer.String())
  	} else {
  		d.SetNull()
  	}
  	return d
  }
docs: // GetResult implements Aggregation interface.