code: func (b *builtinTrim2ArgsSig) evalString(row chunk.Row) (d string, isNull bool, err error) {
  	var str, remstr string
  
  	str, isNull, err = b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return d, isNull, err
  	}
  	remstr, isNull, err = b.args[1].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return d, isNull, err
  	}
  	d = trimLeft(str, remstr)
  	d = trimRight(d, remstr)
  	return d, false, nil
  }
docs: // evalString evals a builtinTrim2ArgsSig, corresponding to trim(str, remstr)
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_trim