code: func (b *builtinUnixTimestampDecSig) evalDecimal(row chunk.Row) (*types.MyDecimal, bool, error) {
  	val, isNull, err := b.args[0].EvalTime(b.ctx, row)
  	if isNull || err != nil {
  		// Return 0 for invalid date time.
  		return new(types.MyDecimal), isNull, nil
  	}
  	t, err := val.Time.GoTime(getTimeZone(b.ctx))
  	if err != nil {
  		return new(types.MyDecimal), false, nil
  	}
  	result, err := goTimeToMysqlUnixTimestamp(t, b.tp.Decimal)
  	return result, err != nil, err
  }
docs: // evalDecimal evals a UNIX_TIMESTAMP(time).
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_unix-timestamp