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Re: in() VS exists()

From: "Bart Degryse" <Bart(dot)Degryse(at)indicator(dot)be>
To: "Julien" <jcigar(at)ulb(dot)ac(dot)be>
Cc: "pgsql-sql" <pgsql-sql(at)postgresql(dot)org>
Subject: Re: in() VS exists()
Date: 2008-03-13 14:12:49
Message-ID: 47D94471.A3DD.0030.0@indicator.be (view raw or flat)
Thread:
Lists: pgsql-sql
I think that just
select count(sp.id) from specimens sp INNER JOIN sequences s on
s.specimen_id = sp.id;
should be enough

>>> Julien <jcigar(at)ulb(dot)ac(dot)be> 2008-03-13 17:10 >>>
If I understood well the query plan, the planner optimize the
IN(SELECT ...) version with a JOIN (line 19-20 of the first paste) :

->  Hash IN Join  (cost=240.95..4011.20 rows=1436 width=4) (actual
time=93.971..201.908 rows=1431 loops=1)
         Hash Cond: ("outer".id = "inner".specimen_id)

so I guess that :

select count(sp.id) from specimens sp where sp.id in (select
specimen_id
from sequences);

is almost the same as :

select count(sp.id) from specimens sp INNER JOIN (select specimen_id
from sequences GROUP BY specimen_id) as foo on foo.specimen_id =
sp.id;

?

Thanks,
Julien

On Thu, 2008-03-13 at 14:46 +0100, Bart Degryse wrote:
> The chapter on indexes in the manual should give you a pretty good
> idea on the why.
> IN and EXISTS are not the only possibilities, you can also use inner
> or outer joins.
> Which solution performs best depends on the data, the database
> version, the available indexes, ...
> 
> >>> Julien <jcigar(at)ulb(dot)ac(dot)be> 2008-03-13 15:47 >>>
> Hello,
> 
> Does anyone has an idea why sometimes:
> - select ... where ... in (select ...)
> is faster than :
> - select ... where ... exists(select ...)
> and sometimes it's the opposite ?
> 
> I had such a situation, I've pasted the queries on:
> http://rafb.net/p/KXNZ6892.html and http://rafb.net/p/jvo5DO38.html 
> 
> It's running PostgreSQL 8.1 with an effective_cache_size of 30000.
> 
> specimens.id is the primary key and there are indexes on
> sequences(specimen_id) and specimen_measurements(specimen_id)
> 
> Is there a general "rule" to know when to use the in() version and
> when
> to use the exists() version ? Is it true to say that the exists()
> version is more scalable (with many rows) than the in() version
(from
> the little tests I made it seems the case) ?
> 
> Thanks,
> Julien
> 
> -- 
> Julien Cigar
> Belgian Biodiversity Platform
> http://www.biodiversity.be ( http://www.biodiversity.be/ )
> Université Libre de Bruxelles (ULB)
> Campus de la Plaine CP 257
> Bâtiment NO, Bureau 4 N4 115C (Niveau 4)
> Boulevard du Triomphe, entrée ULB 2
> B-1050 Bruxelles
> Mail: jcigar(at)ulb(dot)ac(dot)be 
> @biobel: http://biobel.biodiversity.be/person/show/471 
> Tel : 02 650 57 52
> 
> 
> -- 
> Sent via pgsql-sql mailing list (pgsql-sql(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-sql 
> 
-- 
Julien Cigar
Belgian Biodiversity Platform
http://www.biodiversity.be ( http://www.biodiversity.be/ )
Université Libre de Bruxelles (ULB)
Campus de la Plaine CP 257
Bâtiment NO, Bureau 4 N4 115C (Niveau 4)
Boulevard du Triomphe, entrée ULB 2
B-1050 Bruxelles
Mail: jcigar(at)ulb(dot)ac(dot)be 
@biobel: http://biobel.biodiversity.be/person/show/471 
Tel : 02 650 57 52


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