From: Rikard Pavelic <rikard.pavelic@zg.htnet.hr>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: self join revisited
Date: Wed, 01 Apr 2009 20:41:49 +0200
Message-ID: <49D3B56D.9050802@zg.htnet.hr> (raw)
In-Reply-To: <7884.1238606350@sss.pgh.pa.us>
References: <49D3969F.4030701@zg.htnet.hr>
<7884.1238606350@sss.pgh.pa.us>
Tom Lane wrote:
> Rikard Pavelic <rikard.pavelic@zg.htnet.hr> writes:
>
>> It would be great if Postgres could rewrite this query
>>
>
> AFAICS those queries are not going to produce the same results,
> so Postgres would be 100% incorrect to rewrite it like that for you.
>
> (If they do produce the same results, it would depend on a bunch
> of assumptions you have not stated.)
>
> regards, tom lane
>
Can I try again? :)
How hard would it be to teach the planner about preserving uniqueness of
relations in subqueries?
And using that information to remove unnecessary self joins on unique sets?
I can try to rewrite some queries to test it on real data for how much
gain it would provide.
Regards,
Rikard
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To: pgsql-performance@postgresql.org
Cc: rikard.pavelic@zg.htnet.hr, tgl@sss.pgh.pa.us
Subject: Re: self join revisited
In-Reply-To: <49D3B56D.9050802@zg.htnet.hr>
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