Re: LOB objects

  • From: Laimutis.Nedzinskas@xxxxxx
  • To: Joel.Patterson@xxxxxxxxxxx
  • Date: Thu, 17 Mar 2011 20:28:59 +0200


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.

Brgds, Laimis N


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  From:       <Joel.Patterson@xxxxxxxxxxx>                                      
  To:         <oracle-l@xxxxxxxxxxxxx>                                          
  Date:       2011.03.17 20:09                                                  
  Subject:    LOB objects                                                       

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?

Joel Patterson
Database Administrator
904 727-2546


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