code: func (ff *firstRowFunction) StreamUpdate(row []types.Datum, ectx context.Context) error {
  	ctx := ff.getStreamedContext()
  	if ctx.GotFirstRow {
  		return nil
  	}
  	if len(ff.Args) != 1 {
  		return errors.New("Wrong number of args for AggFuncFirstRow")
  	}
  	value, err := ff.Args[0].Eval(row, ectx)
  	if err != nil {
  		return errors.Trace(err)
  	}
  	ctx.Value = value
  	ctx.GotFirstRow = true
  	return nil
  }
docs: // StreamUpdate implements AggregationFunction interface.