code: func (b *builtinLowerSig) evalString(row chunk.Row) (d string, isNull bool, err error) {
  	d, isNull, err = b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return d, isNull, err
  	}
  
  	if types.IsBinaryStr(b.args[0].GetType()) {
  		return d, false, nil
  	}
  
  	return strings.ToLower(d), false, nil
  }
docs: // evalString evals a builtinLowerSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_lower