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Re: Useless sort by

From: Gaetano Mendola <mendola(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Useless sort by
Date: 2010-09-13 17:09:11
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Lists: pgsql-performance
On Mon, Sep 13, 2010 at 6:48 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 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.

I see your point, but some functions like:  unique, count are not affected
by the order of values fed, and I don't think either that unique has to
give out the unique values in the same fed order.

Gaetano Mendola


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