code: func (col *Column) EvalString(ctx sessionctx.Context, row chunk.Row) (string, bool, error) {
  	if row.IsNull(col.Index) {
  		return "", true, nil
  	}
  
  	// Specially handle the ENUM/SET/BIT input value.
  	if col.GetType().Hybrid() {
  		val := row.GetDatum(col.Index, col.RetType)
  		res, err := val.ToString()
  		return res, err != nil, err
  	}
  
  	val := row.GetString(col.Index)
  	if ctx.GetSessionVars().StmtCtx.PadCharToFullLength && col.GetType().Tp == mysql.TypeString {
  		valLen := len([]rune(val))
  		if valLen < col.RetType.Flen {
  			val = val + strings.Repeat(" ", col.RetType.Flen-valLen)
  		}
  	}
  	return val, false, nil
  }
docs: // EvalString returns string representation of Column.