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