From: Thom Brown <thombrown@gmail.com>
To: pgsql-sql@postgresql.org
Subject: Re: Lag and lead window functions order by weirdness
Date: Mon, 12 Oct 2009 08:09:42 +0100
Message-ID: <bddc86150910120009p3ae9d578x2dac139a71e4d045@mail.gmail.com> (raw)
In-Reply-To: <bddc86150910091726s704277f9u7ddcead9874889b3@mail.gmail.com>
References: <bddc86150910091726s704277f9u7ddcead9874889b3@mail.gmail.com>
2009/10/10 Thom Brown <thombrown@gmail.com>
> I've had a look at examples of lag and lead window functions with order by
> inside the OVER clause, and I'm confused as to why it influences the overall
> order in the output.
> For example (for a table called category with incrementing id numbers up to
> 26):
>
> SELECT id, lag(id) OVER (ORDER BY id DESC) FROM category;
>
> This would yield:
>
> id lag
> 26 NULL
> 25 26
> 24 25
>
> This is the equivalent of what we would get with:
> SELECT id, lead(id) OVER (ORDER BY id ASC) FROM category ORDER BY id DESC;
>
> I would expect the row order in the results not to be influenced by the
> OVER clause's ORDER BY as I thought that was just to determine how the
> aggregate window function's values would be output. It's almost as if the
> main part of the query has inherited it's order from the OVER clause.
>
> Am I missing something here?
>
> Thanks
>
> Thom
>
Having a look around, it looks as if Postgres might be misbehaving.
According to this page,
http://my.safaribooksonline.com/0596004818/sqlnut2-CHP-4-SECT-3, the ORDER
BY in the window function's clause shouldn't be having this ordering effect:
"Furthermore, the order within these groups is defined by an ordering
clause, but that order only affects function evaluation, and has no effect
on the order in which rows are returned by the query."
The behaviour is unexpected from my perspective, but obviously there are
workarounds. Is anyone able to confirm any of this?
Thom
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-sql@postgresql.org
Cc: thombrown@gmail.com
Subject: Re: Lag and lead window functions order by weirdness
In-Reply-To: <bddc86150910120009p3ae9d578x2dac139a71e4d045@mail.gmail.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox