From: Dario Pudlo <dariop@unitech.com.ar>
To: pgsql-performance@postgresql.org
Subject: join and query planner
Date: Wed, 6 Jul 2005 18:54:02 -0300
Message-ID: <MHEDJHCKDNOEHJKHIOCJIEFKCEAA.dariop@unitech.com.ar> (raw)
In-Reply-To: <MHEDJHCKDNOEHJKHIOCJKEFHCEAA.dario_d_s@unitech.com.ar>
(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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To: pgsql-performance@postgresql.org
Cc: dariop@unitech.com.ar
Subject: Re: join and query planner
In-Reply-To: <MHEDJHCKDNOEHJKHIOCJIEFKCEAA.dariop@unitech.com.ar>
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