Subject: Re: in() VS exists()
From: Julien <jcigar@ulb.ac.be>
To: Bart Degryse <Bart.Degryse@indicator.be>
Cc: pgsql-sql <pgsql-sql@postgresql.org>
In-Reply-To: <47D93E36.A3DD.0030.0@indicator.be>
References: <1205419644.3240.19.camel@frodon.be-bif.ulb.ac.be>
	<47D93E36.A3DD.0030.0@indicator.be>
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Date: Thu, 13 Mar 2008 17:10:00 +0100
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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@ulb.ac.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
> 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@ulb.ac.be
> @biobel: http://biobel.biodiversity.be/person/show/471
> Tel : 02 650 57 52
> 
> 
> -- 
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-- 
Julien Cigar
Belgian Biodiversity Platform
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@ulb.ac.be
@biobel: http://biobel.biodiversity.be/person/show/471
Tel : 02 650 57 52