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From: John Huttley <John@mib-infotech.co.nz>
To: Andrew Sullivan <ajs@commandprompt.com>
To: pgsql-performance@postgresql.org
Subject: Re: Slow updates, poor IO
Date: Sat, 27 Sep 2008 11:03:38 +1200
Message-ID: <48DD6A4A.10209@mib-infotech.co.nz> (raw)
In-Reply-To: <20080926131556.GA1005@commandprompt.com>
References: <48DBE587.3050406@mib-infotech.co.nz>
	<20080926131556.GA1005@commandprompt.com>

Hi Andrew,
There are two problems.
The first is the that if there is a table with a index and an update is 
performed on a non indexed field,
the index is still re indexed. this is part of the trade-offs of MVCC.
Apparently this is documented under 'MVCC' in the manual. It should be 
documented under 'performance'

We should reasonably expect that the total amount of IO will go up, over 
a non-indexed table.

The second thing is that the disk IO throughput goes way down.

This is not an issue with MVCC, as such, except that it exposes the 
effect of a write to an indexed field.
--even if you don't expect it.

--john

Andrew Sullivan wrote:
> Hi,
>
> On Fri, Sep 26, 2008 at 07:24:55AM +1200, John Huttley wrote:
>   
>> I've just had an interesting encounter with the slow full table update 
>> problem that is inherent with MVCC
>>     
>
> Quite apart from the other excellent observations in this thread, what
> makes you think this is an MVCC issue exactly?
>
> A
>
>   

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