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Re: ??: Postgresql update op is very very slow

From: Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>
To: Holger Hoffstaette <holger(at)wizards(dot)de>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: ??: Postgresql update op is very very slow
Date: 2008-06-26 13:16:25
Message-ID: 486396A9.1030208@postnewspapers.com.au (view raw or flat)
Thread:
Lists: pgsql-performance
Holger Hoffstaette wrote:
> Hi -
> 
> I have been following this thread and find some of the recommendations
> really surprising. I understand that MVCC necessarily creates overhead,
> in-place updates would not be safe against crashes etc. but have a hard
> time believing that this is such a huge problem for RDBMS in 2008. How do
> large databases treat mass updates? AFAIK both DB2 and Oracle use MVCC
> (maybe a different kind?) as well, but I cannot believe that large updates
> still pose such big problems.
> Are there no options (algorithms) for adaptively choosing different
> update strategies that do not incur the full MVCC overhead?

I think Pg already does in place updates, or close, if the tuples being 
replaced aren't referenced by any in-flight transaction. I noticed a 
while ago that if I'm doing bulk load/update work, if there aren't any 
other transactions no MVCC bloat seems to occur and updates are faster.

I'd be interested to have this confirmed, as I don't think I've seen it 
documented anywhere. Is it a side-effect/benefit of HOT somehow?

--
Craig Ringer


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