Re: Flush UNDOTBS?
- From: "Norman Dunbar" <norman.dunbar@xxxxxxxxxxxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>, <cemail_219@xxxxxxxxxxx>
- Date: Tue, 07 Feb 2006 07:56:38 +0000
Morning J Dex,
Norman Dunbar.
Contract Oracle DBA.
Rivers House, Leeds.
Internal : 7 28 2051
External : 0113 231 2051
>>> "J. Dex" <cemail_219@xxxxxxxxxxx> 02/07/06 03:10am >>>
>> How can the UNDOTBS be flushed so that it isn't used?
You simply don't need to flush the UNDO tablespace. It's a wee bit like
the old 'how do I flush TEMP' queries of old.
There are undo extents created in the UNDO tablespace. Once used, they
get retained for a while as set by UNDO_RETENTION, the time is in
seconds, then they expire.
Until an extent is flagged as expired, it will not be overwritten as
long as there are other expired extents to be reused, or free space to
create new extents.
Extents currently in use will show up as active.
Try this :
SELECT tablespace_name, status, COUNT(*) AS HOW_MANY
FROM dba_undo_extents
GROUP BY tablespace_name, status;
You might get something like the following :
TABLESPACE_NAME STATUS HOW_MANY
------------------------------ --------- ----------
UNDO EXPIRED 87
UNDO UNEXPIRED 187
UNDO ACTIVE 7
So as long as there are some EXPIRED and/or UNEXPIRED you will not run
out of UNDO - unless you run the 'update from hell and fill' the entire
tablespace.
From the above, I've got 281 undo extents already, 7 are currently
being used for something, 187 are being retained - just in case - and 87
have expired and can be reused if more active extents are needed.
Oracle will reuse unexpired extents if it has to, so the UNDO_RETENTION
setting is only advisory.
HTH
Cheers,
Norman.
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