From: Tom Lane <tgl@sss.pgh.pa.us>
To: lutzeb@aeccom.com
Cc: Greg Stark <gsstark@mit.edu>
Cc: pgsql-performance@postgresql.org
Cc: john@arbash-meinel.com
Subject: Re: query produces 1 GB temp file
Date: Sun, 06 Feb 2005 12:16:54 -0500
Message-ID: <4859.1107710214@sss.pgh.pa.us> (raw)
In-Reply-To: <42061B35.6000306@aeccom.com>
References: <20050205182117.0125074@mail2.aeccom.com>
<87y8e2kb8j.fsf@stark.xeocode.com>
<42061B35.6000306@aeccom.com>
Dirk.Lutzebaeck@t-online.de (Dirk Lutzebaeck) writes:
> SELECT DISTINCT ON (df.val_9, df.created, df.flatid) df.docindex,
> df.flatobj, bi.oid, bi.en
> FROM bi,df
> WHERE bi.rc=130170467
> ...
> ORDER BY df.val_9 ASC, df.created DESC
> LIMIT 1000 OFFSET 0
Just out of curiosity, what is this query supposed to *do* exactly?
It looks to me like it will give indeterminate results. Practical
uses of DISTINCT ON generally specify more ORDER BY columns than
there are DISTINCT ON columns, because the extra columns determine
which rows have priority to survive the DISTINCT filter. With the
above query, you have absolutely no idea which row will be output
for a given combination of val_9/created/flatid.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, lutzeb@aeccom.com, gsstark@mit.edu, john@arbash-meinel.com
Subject: Re: query produces 1 GB temp file
In-Reply-To: <4859.1107710214@sss.pgh.pa.us>
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