code: func (c *replaceFunctionClass) fixLength(args []Expression) int {
  	charLen := args[0].GetType().Flen
  	oldStrLen := args[1].GetType().Flen
  	diff := args[2].GetType().Flen - oldStrLen
  	if diff > 0 && oldStrLen > 0 {
  		charLen += (charLen / oldStrLen) * diff
  	}
  	return charLen
  }
docs: // fixLength calculate the Flen of the return type.