Re: Do AGGREGATES consistently use sort order?

stark(at)enterprisedb(dot)com (Gregory Stark) writes:
> "Webb Sprague" <webb(dot)sprague(at)gmail(dot)com> writes:
>
>> I can always count on (note the order name):
>>
>> \a
>> oregon_2007_08_20=# select array_accum(name) from (select name from
>> placenames where desig='crater' order by name desc) a;
>> array_accum
>> {"Yapoah Crater","West Crater","Twin Craters","Timber Crater","Red
>> Crater","Newberry Crater","Nash Crater","Mount Mazama","Millican
>> Crater","Little Nash Crater","Le Conte Crater","Jordan
>> Craters","Diamond Craters","Coffeepot Crater","Cayuse Crater","Black
>> Crater","Big Hole","Belknap Crater"}
>> (1 row)
>>
>> I am interested in stitching a line out of points in postgis, but
>> the order/aggregate thing is a general question.
>
> Yes.
>
> You can even do this with GROUP BY as long as the leading columns of
> the ORDER BY inside the subquery exactly matches the GROUP BY
> columns.
>
> In theory we can't promise anything about future versions of
> Postgres but there are lots of people doing this already so if ever
> this was lost there would probably be some new explicit way to
> achieve the same thing.

Is there not some risk that the query planner might choose to do
hash-based accumulation could discard the subquery's ordering?

Under the visible circumstances, it's unlikely, but isn't it possible
for the aggregation to pick hashing and make a hash of this?
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