code: func (b *builtinUnixTimestampCurrentSig) evalInt(row chunk.Row) (int64, bool, error) {
  	var nowTs = &b.ctx.GetSessionVars().StmtCtx.NowTs
  	if nowTs.Equal(time.Time{}) {
  		*nowTs = time.Now()
  	}
  	dec, err := goTimeToMysqlUnixTimestamp(*nowTs, 1)
  	if err != nil {
  		return 0, true, err
  	}
  	intVal, err := dec.ToInt()
  	terror.Log(err)
  	return intVal, false, nil
  }
docs: // evalInt evals a UNIX_TIMESTAMP().
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_unix-timestamp