From: Dawid Kuroczko <qnex42@gmail.com>
To: PFC <lists@peufeu.com>
Cc: Christian Kratzer <ck@cksoft.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Cc: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Big IN() clauses etc : feature proposal
Date: Tue, 9 May 2006 18:43:23 +0200
Message-ID: <758d5e7f0605090943p1bbfdae2p4cb28ba128288316@mail.gmail.com> (raw)
In-Reply-To: <op.s8916gm6cigqcu@apollo13>
References: <445F7FF8.6070707@comcast.net>
<1147110142.32369.7.camel@archimedes>
<20060508175013.GY99570@pervasive.com>
<28288.1147111196@sss.pgh.pa.us>
<op.s89zl30wcigqcu@apollo13>
<20060509105214.P90693@vesihiisi.cksoft.de>
<op.s8916gm6cigqcu@apollo13>
On 5/9/06, PFC <lists@peufeu.com> wrote:
> > You might consider just selecting your primary key or a set of
> > primary keys to involved relations in your search query. If you
> > currently use "select *" this can make your result set very large.
> >
> > Copying all the result set to the temp. costs you additional IO
> > that you propably dont need.
>
> It is a bit of a catch : I need this information, because the purpose of
> the query is to retrieve these objects. I can first store the ids, then
> retrieve the objects, but it's one more query.
>
> > Also you might try:
> > SELECT * FROM somewhere JOIN result USING (id)
> > Instead of:
> > SELECT * FROM somewhere WHERE id IN (SELECT id FROM result)
>
> Yes you're right in this case ; however the query to retrieve the owners
> needs to eliminate duplicates, which IN() does.
Well, you can either
SELECT * FROM somewhere JOIN (SELECT id FROM result GROUP BY id) AS
a USING (id);
or even, for large number of ids:
CREATE TEMPORARY TABLE result_ids AS SELECT id FROM RESULT GROUP BY id;
SELECT * FROM somewhere JOIN result_ids USING (id);
> > On the other hand if your search query runs in 10ms it seems to be fast
> > enough for you to run it multiple times. Theres propably no point in
> > optimizing anything in such case.
>
> I don't think so :
> - 10 ms is a mean time, sometimes it can take much more time, sometimes
> it's faster.
> - Repeating the query might yield different results if records were added
> or deleted in the meantime.
You may SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
though locking might bite you. :)
> - Complex search queries have imprecise rowcount estimates ; hence the
> joins that I would add to them will get suboptimal plans.
>
> Using a temp table is really the cleanest solution now ; but it's too
> slow so I reverted to generating big IN() clauses in the application.
A thought, haven't checked it though, but...
You might want to use PL to store values, say PLperl, or even C, say:
create or replace function perl_store(name text, val int) returns void
as $$ my $name = shift; push @{$foo{$name}}, shift; return $$ LANGUAGE
plperl;
select perl_store('someids', id) from something group by id;
(you may need to warp it inside count())
Then use it:
create or replace function perl_retr(name text) returns setof int as
$$ my $name = shift; return $foo{$name} $$ LANGUAGE plperl;
select * from someother join perl_retr('someids') AS a(id) using (id);
All is in the memory. Of course, you need to do some cleanup, test it,
etc, etc, etc. :)
Should work faster than a in-application solution :)
Regards,
Dawid
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To: pgsql-performance@postgresql.org
Cc: qnex42@gmail.com, lists@peufeu.com, ck@cksoft.de, tgl@sss.pgh.pa.us, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Big IN() clauses etc : feature proposal
In-Reply-To: <758d5e7f0605090943p1bbfdae2p4cb28ba128288316@mail.gmail.com>
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