Re: Improving a simple query?

От: Steve Wampler
Тема: Re: Improving a simple query?
Дата: ,
Msg-id: 20030713204610.GA18123@weaver.tuc.noao.edu
(см: обсуждение, исходный текст)
Ответ на: Re: Improving a simple query?  ("Richard Huxton")
Ответы: Re: Improving a simple query?  (Hannu Krosing)
Re: Improving a simple query?  ()
Re: Improving a simple query?  (Chris Bowlby)
Список: pgsql-performance

Скрыть дерево обсуждения

Improving a simple query?  (Steve Wampler, )
 Re: Improving a simple query?  ("Richard Huxton", )
  Re: Improving a simple query?  (Steve Wampler, )
   Re: Improving a simple query?  (Hannu Krosing, )
   Re: Improving a simple query?  (, )
   Re: Improving a simple query?  (Chris Bowlby, )
    Re: Improving a simple query?  (Chris Bowlby, )
 Re: Improving a simple query?  (, )

On Sun, Jul 13, 2003 at 08:09:17PM +0100, Richard Huxton wrote:
> > I'm not an SQL or PostgreSQL expert.
> >
> > I'm getting abysmal performance on a nested query and
> > need some help on finding ways to improve the performance:
> [snip]
> >  select * from attributes_table where id in (select id from
> >       attributes where (name='obsid') and (value='oid00066'));
>
> This is the classic IN problem (much improved in 7.4 dev I believe). The
> recommended approach is to rewrite the query as an EXISTS form if
> possible. See the mailing list archives for plenty of examples.
>
> Could you not rewrite this as a simple join though?

Hmmm, I don't see how.  Then again, I'm pretty much the village
idiot w.r.t. SQL...

The inner select is locating a set of (2049) ids (actually from
the same table, since 'attributes' is just a view into
'attributes_table').  The outer select is then locating all
records (~30-40K) that have any of those ids.  Is that really
something a JOIN could be used for?

-Steve
--
Steve Wampler -- 
Quantum materiae materietur marmota monax si marmota
                    monax materiam possit materiari?


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