From: Andrei Lepikhov <a.lepikhov@postgrespro.ru>
To: Richard Guo <guofenglinux@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Korotkov <aekorotkov@gmail.com>
Cc: zuming.jiang@inf.ethz.ch, pgsql-bugs@lists.postgresql.org, PG Bug reporting form <noreply@postgresql.org>
Subject: Re: BUG #18187: Unexpected error: "variable not found in subplan target lists" triggered by JOIN
Date: Thu, 30 Nov 2023 14:21:50 +0700
Message-ID: <85d91f13-b6d3-4d5c-8866-f2d9533c042a@postgrespro.ru> (raw)
In-Reply-To: <CAMbWs48UBgxyzNKgu7ZrWDGy-6y33+uC8znyDceRzaDb=+XOcw@mail.gmail.com>
References: <18187-831da249cbd2ff8e@postgresql.org>
<491881ef-f1c7-4a86-b5dd-dcad55a88ab4@postgrespro.ru>
<CAMbWs4_yrqMYXd+zXc4Y6KjnyA8xPZE6aiAW6k2Y10WaNhwRhw@mail.gmail.com>
<CAMbWs4-ns73VF9gi37q61G3dS6Xuos+HtryMaBh37WQn=BsaJw@mail.gmail.com>
<CAMbWs4_WeARU0oYH1sMn03VYzw-qZQ_xwUNq7r6TbDTH+VgXng@mail.gmail.com>
<3883031.1701106936@sss.pgh.pa.us>
<CAMbWs48UBgxyzNKgu7ZrWDGy-6y33+uC8znyDceRzaDb=+XOcw@mail.gmail.com>
On 28/11/2023 14:03, Richard Guo wrote:
>
> On Tue, Nov 28, 2023 at 1:42 AM Tom Lane <tgl@sss.pgh.pa.us
> <mailto:tgl@sss.pgh.pa.us>> wrote:
>
> Richard Guo <guofenglinux@gmail.com <mailto: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 <http://t1.id; = t2.id <http://t2.id;
> ) on i.f0 = t1.id <http://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.
Agree. PlannerInfo is a hot structure where many new features try to
save some data. The history of this patch has shown, for example,
JoinDomains. Finding field jd_relids, which could contain the link to
the removing relation, was tricky.
I still have no idea how to check it internally except by attaching a
comment to the PlannerInfo structure.
--
regards,
Andrei Lepikhov
Postgres Professional
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To: pgsql-bugs@postgresql.org
Cc: a.lepikhov@postgrespro.ru, guofenglinux@gmail.com, tgl@sss.pgh.pa.us, aekorotkov@gmail.com, noreply@postgresql.org
Subject: Re: BUG #18187: Unexpected error: "variable not found in subplan target lists" triggered by JOIN
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