From: Richard Huxton <dev@archonet.com>
To: Mezei Zoltán <mezei.zoltan@telefor.hu>
Cc: pgsql-performance@postgresql.org
Subject: Re: Deceiding which index to use
Date: Fri, 09 Mar 2007 15:52:23 +0000
Message-ID: <45F182B7.30507@archonet.com> (raw)
In-Reply-To: <45F180D2.5080201@telefor.hu>
References: <45F15F0F.8080403@telefor.hu>
<45F1715E.1050000@archonet.com>
<45F174CF.8010302@telefor.hu>
<45F1775A.8030701@archonet.com>
<45F179AB.7040000@telefor.hu>
<45F17B70.60001@archonet.com>
<45F180D2.5080201@telefor.hu>
Mezei Zoltán wrote:
> Richard Huxton wrote:
>>
>> OK - so the next place to look is the distribution of values for
>> subscriber_id on the output_message_log. Does that have some subscribers
>> with many rows and lots with hardly any?
>>
> Hmm... There are about 1.5k subscribers with 100-200 messages each - all the
> other 19k has an average of 8.9 messages, most of them having only 1 message. I
> think that's exactly the situation you mention...
[snip alter table ... set statistics]
> So if I'm correct: this statistics gathering can be fine tuned, and if i set the
> <num> to 1000 then not only the first 10 subsribers (with most messages) will be
> stored in pg_stats, but the first 1000? Is 1000 a hard-coded
> highest-possible-value? I think it would be best to set that to simething like
> 1800-1900 as I have about that many subscibers with high message count.
There is a cost to increasing the stats values, otherwise it'd already
be set at 1000. In your case I'm not sure if 100-200 vs 8-9 messages is
enough to skew things. Only one way to find out...
--
Richard Huxton
Archonet Ltd
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To: pgsql-performance@postgresql.org
Cc: dev@archonet.com, mezei.zoltan@telefor.hu
Subject: Re: Deceiding which index to use
In-Reply-To: <45F182B7.30507@archonet.com>
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