From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: Re: bad execution plan for subselects containing windowing-function
Date: Thu, 14 Jan 2010 19:03:49 +0100
Message-ID: <20100114180349.GA9962@tux> (raw)
In-Reply-To: <25915.1263490925@sss.pgh.pa.us>
References: <20100114170318.GA7634@tux>
<25448.1263489336@sss.pgh.pa.us>
<20100114173025.GA8675@tux>
<25915.1263490925@sss.pgh.pa.us>
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
--
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)
Kaufbach, Saxony, Germany, Europe. N 51.05082°, E 13.56889°
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To: pgsql-performance@postgresql.org
Cc: akretschmer@spamfence.net
Subject: Re: bad execution plan for subselects containing windowing-function
In-Reply-To: <20100114180349.GA9962@tux>
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