From: Tom Lane <tgl@sss.pgh.pa.us>
To: Gaetano Mendola <mendola@gmail.com>
Cc: pgsql-performance@postgreSQL.org
Subject: Re: Useless sort by
Date: Mon, 13 Sep 2010 12:48:07 -0400
Message-ID: <14518.1284396487@sss.pgh.pa.us> (raw)
In-Reply-To: <4C8E4982.70004@gmail.com>
References: <i6ks04$2jet$1@news.hub.org>
<12103.1284389088@sss.pgh.pa.us>
<4C8E4982.70004@gmail.com>
Gaetano Mendola <mendola@gmail.com> writes:
> Of course I'm not suggesting to take away the "sort by" and give the user
> an unsorted result, I'm asking why the the optimizer in cases like:
> select unique(a) from v_table_with_order_by;
> doesn't takes away the "order by" inside the view and puts it back "rewriting the
> query like this:
> select unique(a) from v_table_without_order_by
> order by a;
That changes the order in which the rows are fed to unique(a). The
principal real-world use for a non-top-level ORDER BY is exactly to
determine the order in which rows are fed to a function, so we will
have a revolt on our hands if we break that.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, mendola@gmail.com
Subject: Re: Useless sort by
In-Reply-To: <14518.1284396487@sss.pgh.pa.us>
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