Re: LOB objects

With LOBs, the undo segments are kept in the same tablespace as the LOB. At least all the way to 10gr2 and I suspect it's the same in 11g although I haven't checked.
So if you plan to do lots of updates, get ready for heavy I/O on that tablespace! In one particularly bad case, we actually changed from using an update to a delete followed by an insert: it was faster! But that was in 9i and things might have improved now.

-- 
Cheers
Nuno Souto
in wet Sydney, Australia
dbvision_at_iinet.net.au

Laimutis.Nedzinskas_at_seb.lt wrote,on my timestamp of 18/03/2011 5:28 AM:


> Hi,

>

> Some weeks ago I have discussed lob's connection with flashback database

> feature.  Read metalink "Flashback Database Best Practices&  Performance

> [ID 565535.1]"

>

> Randolf Geist advised me another MOS document "LOBS - Storage, Redo and

> Performance Issues [ID 66431.1]]

>

> My personal point of view is this:

>

> LOB's intensively used as "just bigger varchar2's" with default settings

> can(and did) pose performance problems. Looks like LOB's were designed to

> hold large amounts of data:

>

>   - they are not cached by default. That has consequences.

>   - they are not generally stored inline. There exists index structure to

> access them. That has consequences too.

>   - they use direct read/write (if not cached.) Again, there are

> consequences.     My believe is that this is exactly what causes pressure

> on flashback writer (if flashback is enabled in database.) Haven't had a

> time to test some assumptions how it functions but "Flashback Database Best

> Practices&  Performance [ID 565535.1]" points out specifically that LOB

> caching is good for flashback writed and yes, it did help us.

>

> Basically, for me LOB's are evil if used in tables subject to high

> transactions volume.

>

>

>    From:<Joel.Patterson_at_crowley.com>

>

>

> Can anyone point me to some practical pages or experience working with

> CLOBs.   We have some databases that are using CLOBs, and will be using

> them more.

>

> So I am interested it things like ‘do they grow forever’ i.e. deletions

> really don’t happen physically, (if that is so, what to do about it).

> Tuning:  If that is necessary, all aspects, gothas.   Things I could or

> should do?



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