code: func (col *CorrelatedColumn) EvalString(ctx sessionctx.Context, row chunk.Row) (string, bool, error) {
  	if col.Data.IsNull() {
  		return "", true, nil
  	}
  	res, err := col.Data.ToString()
  	resLen := len([]rune(res))
  	if resLen < col.RetType.Flen && ctx.GetSessionVars().StmtCtx.PadCharToFullLength {
  		res = res + strings.Repeat(" ", col.RetType.Flen-resLen)
  	}
  	return res, err != nil, err
  }
docs: // EvalString returns string representation of CorrelatedColumn.