Re: [GENERAL] Equivalence Classes when using IN

On 10 October 2017 at 12:44, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> David Rowley <david.rowley@2ndquadrant.com> writes:
>> If the only reason that is_simple_subquery() rejects subqueries with
>> ORDER BY is due to wanting to keep the order by of a view, then
>> couldn't we make is_simple_subquery() a bit smarter and have it check
>> if the subquery is going to be joined to something else, which likely
>> would destroy the order, or at least it would remove any guarantees of
>> it.
>
> I'm not on board with this.  The assumption is that if the user put an
> ORDER BY there, that means they want that subquery to be computed in that
> order.  It's not for us to decide they didn't mean what they said.
>
> Moreover, there are cases where the ORDER BY would be semantically
> significant, eg if there's a LIMIT or volatile functions or tSRFs
> involved.

Ok, thanks for looking, although, FWIW, LIMIT and tSRFs are still disabled.


-- David Rowley                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services


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