Re: Making Vars outer-join aware

On Mon, Feb 13, 2023 at 7:58 AM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

> The patch broke this query:
>
> select from pg_inherits inner join information_schema.element_types
> right join (select from pg_constraint as sample_2) on true
> on false, lateral (select scope_catalog, inhdetachpending from
> pg_publication_namespace limit 3);
> ERROR: could not devise a query plan for the given query

Thanks for the report! I've looked at it a little bit and traced down
to function have_unsafe_outer_join_ref(). The comment there says

* In practice, this test never finds a problem ...
* ...
* It still seems worth checking
* as a backstop, but we don't go to a lot of trouble: just reject if the
* unsatisfied part includes any outer-join relids at all.

This seems not correct as showed by the counterexample. ISTM that we
need to do the check honestly as what the other comment says

* If the parameterization is only partly satisfied by the outer rel,
* the unsatisfied part can't include any outer-join relids that could
* null rels of the satisfied part.

The NOT_USED part of code is doing this check. But I think we need a
little tweak. We should check the nullable side of related outer joins
against the satisfied part, rather than inner_paramrels. Maybe
something like attached.

However, this test seems to cost some cycles after the change. So I
wonder if it's worthwhile to perform it, considering that join order
restrictions should be able to guarantee there is no problem here.

BTW, here is a simplified query that can trigger this issue on HEAD.

select * from t1 inner join t2 left join (select null as c from t3 left
join t4 on true) as sub on true on true, lateral (select c, t1.a from t5
offset 0 ) ss;

Thanks
Richard