Message-ID: <486396A9.1030208@postnewspapers.com.au>
Date: Thu, 26 Jun 2008 21:16:25 +0800
From: Craig Ringer <craig@postnewspapers.com.au>
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To: Holger Hoffstaette <holger@wizards.de>
Cc: pgsql-performance@postgresql.org
Subject: Re: ??: Postgresql update op is very very slow
References: <473587.97248.qm@web32701.mail.mud.yahoo.com>
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	<004b01c8d671$3dec1630$0544000a@hz.ali.com>
	<486219B6.7050003@enterprisedb.com>
	<005101c8d773$ff81b2c0$0644000a@hz.ali.com>
	<48638939.5080500@enterprisedb.com>
	<pan.2008.06.26.12.40.59.151500@wizards.de>
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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