Re: Chained Rows
- From: "Gaja Krishna Vaidyanatha" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "gajav@xxxxxxxxx" for DMARC)
- To: "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Sun, 27 Apr 2014 11:00:35 -0700 (PDT)
Hi Kenny,
In addition to everything the others have said, if you are on 11.2.0.3, you may
be running into Bug#9373758 on tables that have more than 255 columns. The
flavor of this bug we encountered was that any DML to the table/partitions of
the table, caused the second row piece which was originally in the same block,
to be migrated to a different block. So we went from 2 row pieces in the same
block to 2 row pieces in two different blocks. Issuing a "move partition" fixed
the issue and was the workaround until 11.2.0.4 was available. But we
unfortunately also hit another flavor of the bug in 11.2.0.4.
So bottom line - we engaged in a table re-design to get the number of columns <
255. This was the only surefire way to avoid this issue. Hope this helps!
Cheers,
Gaja
Gaja Krishna Vaidyanatha,
CEO & Founder, DBPerfMan LLC
http://www.dbperfman.com
http://www.dbcloudman.com
Phone - +1 (650) 743-6060
LinkedIn - http://www.linkedin.com/in/gajakrishnavaidyanatha
Co-author: Oracle Insights:Tales of the Oak Table -
http://www.apress.com/9781590593875
Primary Author: Oracle Performance Tuning 101 - http://www.amzn.com/0072131454
Enabling Exadata, Big Data and Cloud Deployment & Management for Oracle
________________________________
From: Tim Gorman <tim@xxxxxxxxx>
To: oracle-l@xxxxxxxxxxxxx
Sent: Thursday, April 24, 2014 4:52 AM
Subject: Re: Chained Rows
Ken,
Coming into this thread late, so I've not seen your original post
in the thread.
Since the table has so many columns, and if there is the
possibility that not all of those columns are used, have you tried
re-organizing the table so that the most likely NULL columns
trail, and the most likely NOT NULL columns lead? I have a PL/SQL
stored procedure called CARL (file "carl.sql" at
"http://evdbt.com/scripts/";) which is designed to answer the "what if" question
about using "trailing NULL columns" compression, much the same as the
DBMS_COMPRESSION package is designed to answer the "what if" question about
whether or not to use different types of compression (i.e. basic, oltp, hcc,
etc). The big difference is that CARL does not temporarily create a compressed
table as DBMS_COMPRESSION does, but calculates everything using gathered
statistics residing in the DBA_TAB_COLUMNS view, so you'd want good statistics.
It may not help your problem, but it'll tell you pretty quickly
whether it is worth trying.
Hope this helps.
Thanks!
-Tim
On 4/24/2014 3:34 AM, Kenny Payton wrote:
Yes. We are at 8k and I suspect a 16k block size would reduce the rate by
1/2. We have talked about doing this for some time. Unfortunately the most
widely affected areas of the database is 20T of data out of 160T of databases.
Also efficiently managing multiple buffer pools becomes difficult.
>What I find challenging is that I don't see a way to avoid it all together.
>If you have a table with more than 255 columns and you use any of the columns
>beyond 255 this is going to happen at some difficult to predict rate.
>Ideally I would want Oracle to put all row pieces in a single block if they
>would fit. If not then consider them chained and break it up. It seems to
>treat each row piece independently from the start.
>
>On Apr 24, 2014 1:42 AM, "Hans Forbrich" <fuzzy.graybeard@xxxxxxxxx> wrote:
>
>Is this possibly a valid use case for larger tablespace block size?
>>
>>On 23/04/2014 4:29 PM, Kenny Payton wrote:
>>
>>Thanks.
>>>Unfortunately the data and access patterns change pretty frequently. Â
>>>Another reason we find ourselves in this situation.
>>>On Apr 23, 2014 6:02 PM, "Sayan Malakshinov" <xt.and.r@xxxxxxxxx> wrote:
>>>
>>>
>>>>
>>>>On Thu, Apr 24, 2014 at 12:56 AM, Kenny Payton <k3nnyp@xxxxxxxxx> wrote:
>>>>I know a lot of the tricks for avoiding chained rows and migrated rows but
>>>>the only trick I know of to element these intra or inter block chained rows
>>>>is to break the table into multiple tables or the row into multiple rows.Â
>>>>
>>>>Sometimes might be helpful to redefine table with moving less-used columns
>>>>to the ends of rows. It allows to reduce extra-work if most queries use
>>>>only the first columns
>>>>
>>>>
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