Re: plan shape work

On Mon, Sep 8, 2025 at 5:51 AM Richard Guo <guofenglinux@gmail.com> wrote:
> > Plain (not-outer) joins will never be included in a relid set in the
> > first place.
>
> Exactly.  Non-outer joins wouldn't cause Vars to become null, so we
> never include them in the joinrel's relids.

OK. I didn't understand what the rules were there.

> BTW, I'm wondering if we can take outer join relids into account in
> assert_join_preserves_scan_rtis(), which could make the check more
> useful.  A joinrel's relids consists of three parts: the outer plan's
> relids, the inner plan's relids, and the relids of outer joins that
> are calculated at this join.  We already have the first two.  If we
> can find a way to determine the third, we'd be able to assert that:
>
>  outer_relids U inner_relids U outerjoin_relids == joinrel->relids
>
> Determining the third part can be tricky though, especially due to
> outer-join identity 3: the "outerjoin_relids" of one outer join might
> include more than one outer join relids.  But I think this is till
> doable.
>
> (This may not be useful for your overall goal in this patchset, so
> feel free to ignore it if it's not of interest.)

I don't mind doing the work if there's a reasonable and useful way of
accomplishing the goal. However, one concern I have is that it seems
pointless if we're computing outerjoin_relids by essentially redoing
the same computation that set the join's relid set in the first place.
In that case, the cross-check has no real probative value. All it
would be demonstrating is that if you calculate outerjoin_relids twice
using essentially the same methodology, you get the same answer. That
seems like a waste of code to me. If there's a way to calculate
outerjoin_relids using a different methodology than what we used when
populating the joinrelids, that would be interesting. It would be
similar to how the existing code recomputes the outer and inner relids
in a way that can potentially find issues that otherwise would not
have been spotted (such as the Result node case).

Do you have a proposal?

--
Robert Haas
EDB: http://www.enterprisedb.com