Message-Id: <s2dba73f.020@gwmta.wicourts.gov>
Date: Mon, 18 Jul 2005 12:57:31 -0500
From: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
To: <pgsql-performance@postgresql.org>, <dariop@unitech.com.ar>
Subject: Re: join and query planner
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Just out of curiosity, does it do any better with the following?

    SELECT ...
     FROM a
     JOIN b ON (a.key = b.key)
     LEFT JOIN c ON (c.key = a.key)
     LEFT JOIN d ON (d.key=a.key)
     WHERE (b.column <= 100)


>>> "Dario Pudlo" <dariop@unitech.com.ar> 07/06/05 4:54 PM >>>
(first at all, sorry for my english)
Hi.
   - Does "left join" restrict the order in which the planner must join
tables? I've read about join, but i'm not sure about left join...
   - If so: Can I avoid this behavior? I mean, make the planner resolve
the
query, using statistics (uniqueness, data distribution) rather than join
order.

	My query looks like:
	SELECT ...
      FROM a, b,
      LEFT JOIN c ON (c.key = a.key)
      LEFT JOIN d on (d.key=a.key)
      WHERE (a.key = b.key)  AND (b.column <= 100)

      b.column has a lot better selectivity, but planner insist on
resolve
first c.key = a.key.

	Of course, I could rewrite something like:
	SELECT ...
      FROM
           (SELECT ...
            FROM a,b
            LEFT JOIN d on (d.key=a.key)
            WHERE (b.column <= 100)
            )
            as aa
      LEFT JOIN c ON (c.key = aa.key)

	but this is query is constructed by an application with a
"multicolumn"
filter. It's dynamic.
      It means that a user could choose to look for "c.column = 1000".
And
also, combinations of filters.

	So, I need the planner to choose the best plan...

I've already change statistics, I clustered tables with cluster, ran
vacuum
analyze, changed work_mem, shared_buffers...

Greetings. TIA.


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