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join/from_collapse_limit and geqo_threshold default values

From: Vlad Arkhipov <arhipov(at)dc(dot)baikal(dot)ru>
To: pgsql-performance(at)postgresql(dot)org
Subject: join/from_collapse_limit and geqo_threshold default values
Date: 2008-05-23 09:01:16
Message-ID: 483687DC.5030300@dc.baikal.ru (view raw or flat)
Thread:
Lists: pgsql-performance
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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