Re: Chained Rows
Date: Sun, 27 Apr 2014 18:29:58 -0400
Message-ID: <CAEidWqM+M+3_X_zyqnX+152PUL_f5UY20JFsZ8wKmc7EehF+-g_at_mail.gmail.com>
Thanks for the note. I will dig into it tomorrow. In my mind, splitting the table into two doesn't help when you need the 2nd half of the row. You still end up with multiple block reads unless they are clustered and also have to deal with another index.
On Apr 27, 2014 2:01 PM, "Gaja Krishna Vaidyanatha" < dmarc-noreply_at_freelists.org> wrote:
> 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 <http://www.dbperfman.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_at_evdbt.com>
> *To:* oracle-l_at_freelists.org
> *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/" <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_at_gmail.com>
> 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_at_gmail.com> wrote:
>
>
> On Thu, Apr 24, 2014 at 12:56 AM, Kenny Payton <k3nnyp_at_gmail.com> 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
>
>
>
>
>
-- http://www.freelists.org/webpage/oracle-lReceived on Mon Apr 28 2014 - 00:29:58 CEST