Date: Thu, 14 Jan 2010 19:31:39 +0100
From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: Re: bad execution plan for subselects containing
	windowing-function
Message-ID: <20100114183139.GA11334@tux>
References: <20100114170318.GA7634@tux> <25448.1263489336@sss.pgh.pa.us>
	<20100114173025.GA8675@tux> <25915.1263490925@sss.pgh.pa.us>
	<20100114180349.GA9962@tux>
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Andreas Kretschmer <akretschmer@spamfence.net> wrote:
> > 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 ...

I have got it!


test=# select * from values;
 id | value
----+-------
  1 |    10
  2 |    20
  3 |    30
  4 |    40
  5 |    50
  6 |    60
  7 |    70
  8 |    80
  9 |    90
(9 rows)

Time: 0.240 ms
test=*# select id, sum(value) over (order by id) from values where id = 5;
 id | sum
----+-----
  5 |  50
(1 row)

Time: 0.352 ms
test=*# select * from (select id, sum(value) over (order by id) from values) foo where id = 5;
 id | sum
----+-----
  5 | 150
(1 row)

Time: 0.383 ms


Andreas
-- 
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
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