code: func (t *tableCommon) genIndexKeyStr(colVals []types.Datum) (string, error) {
  	// Pass pre-composed error to txn.
  	strVals := make([]string, 0, len(colVals))
  	for _, cv := range colVals {
  		cvs := "NULL"
  		var err error
  		if !cv.IsNull() {
  			cvs, err = types.ToString(cv.GetValue())
  			if err != nil {
  				return "", err
  			}
  		}
  		strVals = append(strVals, cvs)
  	}
  	return strings.Join(strVals, "-"), nil
  }
docs: // genIndexKeyStr generates index content string representation.