From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Gaetano Mendola <mendola(at)gmail(dot)com>, pgsql-performance(at)postgreSQL(dot)org |
Subject: | Re: Useless sort by |
Date: | 2010-09-14 07:10:37 |
Message-ID: | 4C8F1FED.3050902@enterprisedb.com |
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Lists: | pgsql-performance |
On 13/09/10 19:48, Tom Lane wrote:
> Gaetano Mendola<mendola(at)gmail(dot)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.
You could check for volatile functions. I think this could be done
safely. However, it doesn't seem worthwhile, it would be a fair amount
of code, and it's not usually a good idea to put an ORDER BY in a view
or subquery anyway unless you also have volatile functions in there, or
you want to coerce the optimizer to choose a certain plan.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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