Re: Useless sort by

On 13/09/10 19:48, Tom Lane wrote:
> 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.

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