Re: Deceiding which index to use

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From: Mezei Zoltán <mezei.zoltan@telefor.hu>
To: Richard Huxton <dev@archonet.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Deceiding which index to use
Date: Fri, 09 Mar 2007 16:44:18 +0100
Message-ID: <45F180D2.5080201@telefor.hu> (raw)
In-Reply-To: <45F17B70.60001@archonet.com>
References: <45F15F0F.8080403@telefor.hu>
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Richard Huxton wrote:
<blockquote cite="mid:45F17B70.60001@archonet.com" type="cite">
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  <title>Re: [PERFORM] Deceiding which index to use</title>

  <p><font size="2">OK - so the next place to look is the distribution
of values for<br>
subscriber_id on the output_message_log. Does that have some subscribers<br>
with many rows and lots with hardly any?</font></p>
</blockquote>
<small>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...</small><br>
<blockquote cite="mid:45F17B70.60001@archonet.com" type="cite">
  <p><font size="2">If so, you might need to<br>
increase the stats on that column:<br>
  <br>
ALTER TABLE output_message_log ALTER COLUMN subscriber_id SET STATISTICS<br>
&lt;num&gt;;<br>
ANALYSE output_message_log (subscriber_id);<br>
  <br>
The &lt;num&gt; defaults to 10, but can be set as high as 1000. You
want to<br>
try and capture the "big" subscribers.<br>
  </font></p>
</blockquote>
<small>So if I'm correct: this statistics gathering can be fine tuned,
and if i set the &lt;num&gt; 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.<br>
<br>
Zizi<br>
</small>
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