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Re: Flashback database question

From: <hjr.pythian_at_gmail.com>
Date: Wed, 26 Sep 2007 16:23:56 -0700
Message-ID: <1190849036.729202.43240@d55g2000hsg.googlegroups.com>


On Sep 27, 6:19 am, jackzh..._at_gmail.com wrote:
> On Sep 26, 4:05 pm, "Michel Cadot" <micadot{at}altern{dot}org> wrote:
>
>
>
> > <jackzh..._at_gmail.com> a écrit dans le message de news: 1190835448.931165.54..._at_57g2000hsv.googlegroups.com...
> > | 10.2.0.3
> > |
> > | SQL> select sysdate from dual;
> > |
> > | SYSDATE
> > | -------------------
> > | 09/26/2007 15:02:07
> > |
> > | SQL> select OLDEST_FLASHBACK_TIME,RETENTION_TARGET from v
> > | $flashback_database_log;
> > |
> > | OLDEST_FLASHBACK_TI RETENTION_TARGET
> > | ------------------- ----------------
> > | 09/24/2007 23:53:12 360
> > |
> > | Since RETENTION_TARGET is 360 (6 hours), how come the
> > | OLDEST_FLASHBACK_TIME can be way beyond 6 hours?
> > |
> > | Thanks!
> > |
>
> > Oracle did not need to reuse space so it kept older transactions.
>
> > Regards
> > Michel
>
> Thnx, but my question is since RETENTION_TARGET is 6 hours, does this
> mean I only can flashback database up to 6 hrs ago? If so, how come
> the OLDEST_FLASHBACK_TIME is way more than 6 hrs?

The word "target" has a specific meaning in English: things such as arrows, bullets and undo retention are allowed to miss targets by a wide mile!

You've asked for 6 hours of undo retention. Oracle will do its best to achieve this. Undo younger than 6 hours will not be overwritten (or we'll try not to, anyway). Undo older than 6 hours **may** (not "will") be over-written by fresh undo.

But if you do so few transactions that the cavernous wastes of your undo tablespace don't stand a chance in hell of ever being filled to the brim, it stands to reason that undo older than 6 hours can safely be left to sit around un-overwritten. And that means the undo needed to flash back further than 6 hours is still available -and therefore yes, of course you are allowed to use that undo to perform flashback to the oldest time for which undo happens to still be available. Received on Wed Sep 26 2007 - 18:23:56 CDT

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