code: func (s *session) getExecRet(ctx sessionctx.Context, sql string) (string, error) {
  	rows, fields, err := s.ExecRestrictedSQL(ctx, sql)
  	if err != nil {
  		return "", err
  	}
  	if len(rows) == 0 {
  		return "", executor.ErrResultIsEmpty
  	}
  	d := rows[0].GetDatum(0, &fields[0].Column.FieldType)
  	value, err := d.ToString()
  	if err != nil {
  		return "", err
  	}
  	return value, nil
  }
docs: // getExecRet executes restricted sql and the result is one column.
  // It returns a string value.