PL/SQL :: Last Or Oldest Record In A Database Table

I want to fetch oldest record in database table based on llast updated date ; I tried following queries..it gave me different results

SELECT EVENT_ID, RETENTION_TS
FROM ( SELECT EVENT_ID, RETENTION_TS, RANK() OVER (ORDER BY RETENTION_TS) RETENTION_TS_RANK
FROM EVENT )
WHERE RETENTION_TS_RANK <= 1;

select * from EVENT where rowid=(select min(rowid) from EVENT);

SELECT *
FROM (select * from EVENT ORDER BY RETENTION_TS) EVENT2
WHERE rownum <= 1
ORDER BY rownum DESC;

Above query takes like 10 to 20 mins to return me the record.