Re: self join revisited

From: Rikard Pavelic
Subject: Re: self join revisited
Date: ,
Msg-id: 49D3B56D.9050802@zg.htnet.hr
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In response to: Re: self join revisited  (Tom Lane)
Responses: Re: self join revisited  (Robert Haas)
List: pgsql-performance

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self join revisited  (Rikard Pavelic, )
 Re: self join revisited  (Matthew Wakeling, )
 Re: self join revisited  (Tom Lane, )
  Re: self join revisited  (Rikard Pavelic, )
   Re: self join revisited  (Robert Haas, )

Tom Lane wrote:
> Rikard Pavelic <> 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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