Re: Improving a simple query?

От: Chris Bowlby
Тема: Re: Improving a simple query?
Дата: ,
Msg-id: 5.2.1.1.0.20030713232822.00a076c0@mail.hub.org
(см: обсуждение, исходный текст)
Ответ на: Re: Improving a simple query?  (Steve Wampler)
Ответы: 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?  (, )

At 01:46 PM 7/13/03 -0700, Steve Wampler wrote:

  The following left join should work if I've done my select right, you
might want to play with a left versus right to see which will give you a
better result, but this query should help:

  SELECT * FROM attributes_table att LEFT JOIN attributes at ON (at.name =
'obsid' AND at.value = 'oid00066') WHERE att.id = at.id;

>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?
>
>---------------------------(end of broadcast)---------------------------
>TIP 4: Don't 'kill -9' the postmaster



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