Re: BUG #18187: Unexpected error: "variable not found in subplan target lists" triggered by JOIN

On Tue, Nov 28, 2023 at 1:42 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Richard Guo <guofenglinux@gmail.com> writes:
> > After some more thought, I think PHVs should not impose any constraints
> > on removing self joins.  If the removed rel is contained in the relids
> > that a PHV is evaluated/needed at or laterally references, it can just
> > be replaced with the rel that is kept.
>
> I experimented with trying to break this patch using this test case:
>
> create table t (id int primary key, f1 text, f2 text);
> create table i (f0 int);
>
> explain verbose
> select * from i left join (
>   (select *, 1 as x from t tss1) t1 join
>   (select *, 2 as y from t tss2) t2
>   on t1.id = t2.id
> ) on i.f0 = t1.id;


Thanks for the query!


> It seems to successfully remove the self-join between tss1 and tss2,
> but there is something wrong.  If you examine the PlannerInfo just
> after remove_useless_self_joins, you will notice that the two
> PlaceHolderVars (for x and y) in the placeholder_list both have
>
>          :phrels (b 7)
>          :phnullingrels (b)
>
> where beforehand x had
>
>          :phrels (b 6)
>          :phnullingrels (b 5)
>
> and y had
>
>          :phrels (b 7)
>          :phnullingrels (b 5)
>
> It's correct to move both phrels locations to rel 7 (the surviving
> self-joined rel) but what became of the reference to nullingrel 5?
> That seems clearly wrong, since we have not removed the left join.


What you noticed before remove_useless_self_joins seems not right to me.
The PHVs in placeholder_list are supposed to have empty phnullingrels by
convention, as explained in find_placeholder_info.

* By convention, phinfo->ph_var->phnullingrels is always empty, since the
* PlaceHolderInfo represents the initially-calculated state of the
* PlaceHolderVar.


> Much worse, if you look around elsewhere in the data structure,
> particularly at the processed_tlist, you find instances of the
> PlaceHolderVars that have not been updated and still have the
> old values such as ":phrels (b 6)".


Good catch!  We fail to handle PlaceHolderVar case in sje_walker and
replace_varno_walker, which is wrong.  Fixed in v3 patch.


> It's not apparent to me why the cross-checks we have in setrefs.c
> and elsewhere don't detect a problem with this.  But it seems
> clear that remove_useless_self_joins is still several bricks
> shy of a load in terms of fully updating the data structure for a
> join removal.  Probably with some more work to add complication
> to this test case, we could demonstrate an observable failure.


Fair point.  I have the same concern that we still have other loose ends
in SJE updating necessary data structures in self join removal.  And I
noticed that Andres expressed the same concern in [1].  I think we
should come up with some solution to detect/avoid such problems, but I
imagine that that should be a seperate patch.


> BTW, why is it that it seems to prefer to remove the first of
> the two self-joined rels, rather than the second?  That seems
> jarringly bizarre.


Hmm, I'm not sure either.  Alexander and Andrei, could you please share
your insights?

[1]
https://www.postgresql.org/message-id/20231127180705.svzrlahdmqvosqfz%40awork3.anarazel.de

Thanks
Richard