Steve Wampler kirjutas P, 13.07.2003 kell 23:46:
> 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?
There may be some subtle differences, but most likely the 'join' form
wis like this:
select at.*
from attributes_table at,
attributes a
where at.id = a.id
and a.name='obsid'
and a.value='oid00066'
--------------
Hannu
