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Re: row migaration

From: Serge Rielau <srielau_at_ca.ibm.com>
Date: Tue, 27 Dec 2005 23:41:14 -0500
Message-ID: <41ejbjF1e4of3U1@individual.net>


Mladen Gogala wrote:
> On Tue, 27 Dec 2005 21:52:15 -0500, Serge Rielau wrote:
>
>

>>When are the tumbstones/forward pointers cleaned up?

>
>
> Actually, they're not. One can detect them, by using ANALYZE TABLE LIST
> CHAINED ROWS command but they're not cleaned by themselves.
>
>
>>I recall a lot of 
>>debate recently about how Oracle doesn't need reorg. 

>
>
> Oracle doesn't need reorg of an index. The main argument for an occasional
> rebuild of indexes was the myth that oracle doesn't reuse the blocks
> corresponding to deleted keys. It was the truth in Oracle7, no longer so
> in Oracle9. Tables that suffer from excessive row travels and tours will
> need a good reorg in Oracle, too.

Got it, thanks.
>>In DB2 excessive row-migration is fought with reorg....

>
>
> Ignoring or misdiagnosing the problem also works. For instance: if a table
> becomes a hot spot and enormous amount of disk I/O is generated for no
> apparent reason, the conclusion drawn may very well be that you need to
> add more memory in the buffer cache and thus increase the hit ratio. It's
> certainly easier then running analyze and studying the problem carefully.
>
> PS:
> ---
> What are "tumbstones"? I know the story of Wyatt Earp, Tombstone and alike
> but that seems somehow unrelated to the database group.
IIRC the forward pointer to a migrated row is also called a tombstone. I maybe wrong.
It's been a long time since I learned that low level stuff ;-)

Cheers
Serge

-- 
Serge Rielau
DB2 Solutions Development
DB2 UDB for Linux, Unix, Windows
IBM Toronto Lab
Received on Tue Dec 27 2005 - 22:41:14 CST

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