code: func (b *builtinDegreesSig) evalReal(row chunk.Row) (float64, bool, error) {
  	val, isNull, err := b.args[0].EvalReal(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	res := val * 180 / math.Pi
  	return res, false, nil
  }
docs: // evalReal evals a builtinDegreesSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_degrees