Date: Thu, 14 Jan 2010 19:03:49 +0100
From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: Re: bad execution plan for subselects containing
	windowing-function
Message-ID: <20100114180349.GA9962@tux>
References: <20100114170318.GA7634@tux> <25448.1263489336@sss.pgh.pa.us>
	<20100114173025.GA8675@tux> <25915.1263490925@sss.pgh.pa.us>
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Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Andreas Kretschmer <akretschmer@spamfence.net> writes:
> > Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> I see no bug here.  Your second query asks for a much more complicated
> >> computation, it's not surprising it takes longer.
> 
> > But sorry, I disagree. It is the same query with the same result. I can't see
> > how the queries should return different results.
> 
> In the first query
> 
> select id, avg(value) over (partition by value) from values where id = 50 order by id;
> 
> the avg() calculations are being done over only rows with id = 50.  In
> the second query
> 
> select * from (select id, avg(value) over (partition by value) from values  order by id) foo where id = 50;
> 
> they are being done over all rows.  In this particular example you
> happen to get the same result, but that's just because "avg(foo) over
> partition by foo" is a dumb example --- it will necessarily just yield
> identically foo.  In more realistic computations the results would be
> different.

Okay, i believe you now ;-)

I will try to find a case with different results ...

Thx for your fast help!


Andreas
-- 
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