Message-ID: <7332bc61-a9fa-464b-b5a4-4e8e0f764a64@postgrespro.ru>
Date: Tue, 31 Oct 2023 12:05:35 +0700
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Subject: Re: BUG #18170: Unexpected error: no relation entry for relid 3
To: Richard Guo <guofenglinux@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Alexander Korotkov <aekorotkov@gmail.com>,
 Vik Fearing <vik@postgresfriends.org>, zuming.jiang@inf.ethz.ch,
 pgsql-bugs@lists.postgresql.org,
 Alexander Korotkov <akorotkov@postgresql.org>
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From: Andrei Lepikhov <a.lepikhov@postgrespro.ru>
Organization: Postgres Professional
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On 30/10/2023 13:24, Richard Guo wrote:
> 
> On Mon, Oct 30, 2023 at 10:47 AM Andrei Lepikhov 
> <a.lepikhov@postgrespro.ru <mailto:a.lepikhov@postgrespro.ru>> wrote:
> 
>     On 30/10/2023 09:24, Richard Guo wrote:
>      > I also have some concerns about this patch.  It requires that
>      > root->parse remains unchanged during the whole subquery_planner() in
>      > order to work, which is an implicit constraint we did not have
>     before.
> 
>     It is not about unchanged; it is about referencing the same query at
>     the
>     parent and child query blocks. Am I missing something?
> 
> 
> Yeah, that's what I meant.  We need to ensure that root->parse
> references the same Query structure during the whole subquery_planner()
> for this patch to work, which seems hacky, and error-prone for future
> development.  If we really want to do so, at least we need to emphasize
> this point in the comment of subquery_planner().

Okay, let's go another way and try to imagine what additional 
opportunities it will give us in the future? I mean, save unchanged a 
rte->subquery tree and, simultaneously, have some transformed version in 
the subroot->parse field.
I can imagine kind of a subquery replanning procedure in the case we 
realized during higher-level query planning that the underlying subquery 
is not optimal.
Such a picture makes sense, and I can agree with your words. From this 
point of view, all links to the subquery must reference the subroot 
structures, and the patch you have proposed is the best solution. And 
the SJE feature works correctly.
If we don't imagine the picture I have drawn above, it is reasonable to 
save memory and not alter the parse tree while removing unneeded joins. 
I think, in that case, we can use the walker procedure instead of a mutator.

-- 
regards,
Andrei Lepikhov
Postgres Professional