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Re: statement stats extra load?

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Alan McKay <alan(dot)mckay(at)gmail(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: statement stats extra load?
Date: 2009-09-22 06:42:53
Message-ID: 01B4D10F-5725-4AD5-99A3-06830C11A82E@hagander.net (view raw or flat)
Thread:
Lists: pgsql-performance
On 21 sep 2009, at 23.41, Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> Alan McKay wrote:
>>>> And if so, where does that extra load go? ? ?Disk? ?CPU? ?RAM?
>>>
>>> As of 8.4.X the load isn't measurable.
>>
>> Thanks Bruce.  What about 8.3 since that is our current production  
>> DB?
>
> Same.  All statsistics settings that are enabled by default have
> near-zero overhead.  Is there a specific setting you are thinking of?

That's not true at all.

If you have many relations in your cluster that have at some point  
been touched, the starts collector can create a *significant* load on  
the I/o system. I've come across several cases where the only choice  
was to disable the collector completely, even given all the drawbacks  
from that.

8.4 makes this *a lot* better with two new features. One enabled by  
default (write stats file on demand) and one you have to enable  
manually (stats file location). Using both these together can pretty  
much get rid of the issue, but there's no way in 8.3.

/Magnus


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