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Message-ID: <483687DC.5030300@dc.baikal.ru>
Date: Fri, 23 May 2008 18:01:16 +0900
From: Vlad Arkhipov <arhipov@dc.baikal.ru>
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To: pgsql-performance@postgresql.org
Subject: join/from_collapse_limit and geqo_threshold default values
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I wonder why join_collapse_limit default values is set to 8 but 
geqo_threshold is 12. Optimizer doesn't change the order of JOIN's of 
queries that contains from 8 to 11 tables. Why it's 'wise' decision as 
documentation says?

from_collapse_limit (integer)

    The planner will merge sub-queries into upper queries if the
    resulting FROM list would have no more than this many items. Smaller
    values reduce planning time but might yield inferior query plans.
    The default is eight. It is usually wise to keep this less than
    geqo_threshold
    <http://www.postgresql.org/docs/8.3/static/runtime-config-query.html#GUC-GEQO-THRESHOLD>.
    For more information see Section 14.3
    <http://www.postgresql.org/docs/8.3/static/explicit-joins.html>.


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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
</head>
<body bgcolor="#ffffff" text="#000000">
I wonder why join_collapse_limit default values is set to 8 but
geqo_threshold is 12. Optimizer doesn't change the order of JOIN's of
queries that contains from 8 to 11 tables. Why it's 'wise' decision as
documentation says?<br>
<dl>
  <dt><tt class="VARNAME">from_collapse_limit</tt> (<tt class="TYPE">integer</tt>)</dt>
  <dd>
    <p> The planner will merge sub-queries into upper queries if the
resulting <tt class="LITERAL">FROM</tt>
list would have no more than this many items. Smaller values reduce
planning time but might yield inferior query plans. The default is
eight. It is usually wise to keep this less than <a
 href="http://www.postgresql.org/docs/8.3/static/runtime-config-query.html#GUC-GEQO-THRESHOLD">geqo_threshold</a>.
For more information see <a
 href="http://www.postgresql.org/docs/8.3/static/explicit-joins.html">Section
14.3</a>.</p>
  </dd>
</dl>
</body>
</html>

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