From: Sebastian Hennebrueder <usenet@laliluna.de>
To: pgsql-performance@postgresql.org
Subject: Re: Query plan for very large number of joins
Date: Fri, 03 Jun 2005 00:23:55 +0200
Message-ID: <429F86FB.9020103@laliluna.de> (raw)
In-Reply-To: <14834.1117729586@sss.pgh.pa.us>
References: <17165144.1117725925709.JavaMail.tomcat@iecsai19>
<429F2D94.3030805@archonet.com>
<14834.1117729586@sss.pgh.pa.us>
Tom Lane schrieb:
>Richard Huxton <dev@archonet.com> writes:
>
>
>>philb@vodafone.ie wrote:
>>
>>
>>>I am using PostgreSQL (7.4) with a schema that was generated
>>>automatically (using hibernate). The schema consists of about 650
>>>relations. One particular query (also generated automatically)
>>>consists of left joining approximately 350 tables.
>>>
>>>
>
>
>
>>May I be the first to offer an "ouch"!
>>
>>
>
>Seconded.
>
>
>
>>However, I'm not sure how much leeway there is in
>>planning a largely left-joined query.
>>
>>
>
>Not much. The best hope for a better result is to order the LEFT JOIN
>clauses in a way that will produce a good plan.
>
>
If this is the best way, you should consider to use an sql query and not
the hibernate ql language in this case. This is possible with Hibernate!
I suppose you could also consider a view in Postgre and let Hibernate
read from this view. This is also possible.
>One thought is that I am not sure I believe the conclusion that planning
>is taking only 36 ms; even realizing that the exclusive use of left
>joins eliminates options for join order, there are still quite a lot of
>plans to consider. You should try both EXPLAIN and EXPLAIN ANALYZE
>from psql and see how long each takes. It'd also be interesting to keep
>an eye on how large the backend process grows while doing this --- maybe
>it's being driven into swap.
>
>Also: I'm not sure there *is* such a thing as a good plan for a 350-way
>join. It may be time to reconsider your data representation. If
>Hibernate really forces this on you, it may be time to reconsider your
>choice of tool.
>
> regards, tom lane
>
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--
Kind Regards / Viele Grüße
Sebastian Hennebrueder
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To: pgsql-performance@postgresql.org
Cc: usenet@laliluna.de
Subject: Re: Query plan for very large number of joins
In-Reply-To: <429F86FB.9020103@laliluna.de>
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