code: func (cf *concatFunction) GetGroupResult(groupKey []byte) (d types.Datum) {
  	ctx := cf.getContext(groupKey)
  	if ctx.Buffer != nil {
  		d.SetString(ctx.Buffer.String())
  	} else {
  		d.SetNull()
  	}
  	return d
  }
docs: // GetGroupResult implements AggregationFunction interface.