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Re: Slow updates, poor IO

From: John Huttley <John(at)mib-infotech(dot)co(dot)nz>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Slow updates, poor IO
Date: 2008-09-27 22:33:56
Message-ID: 48DEB4D4.1020100@mib-infotech.co.nz (view raw or flat)
Thread:
Lists: pgsql-performance

Scott Marlowe wrote:
> On Fri, Sep 26, 2008 at 5:03 PM, John Huttley <John(at)mib-infotech(dot)co(dot)nz> wrote:
>   
>> 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.
>>     
>
> I assume you mean updated, not reindexed, as reindexed has a different
> meaning as regards postgresql.  Also, this is no longer true as of
> version 8.3.  If you're updating non-indexed fields a lot and you're
> not running 8.3 you are doing yourself a huge disservice.
>
>   

Yes sorry, I mean all indexes are updated even when the updated field is 
not indexed.
I'm running 8.3.3
>> this is part of the trade-offs of MVCC.
>>     
>
> was...  was a part of the trade-offs.
>
>   
You are thinking of HOT?
I don't think it applies in the case of full table updates??

>> 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.
>>     
>
> It's really an effect of parallel updates / writes / accesses, and is
> always an issue for a database running on a poor storage subsystem.  A
> db with a two drive mirror set is always going to be at a disadvantage
> to one running on a dozen or so drives in a RAID-10
>
>   
Oh well, I'm forever going to be disadvantaged.


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