Message-ID: <1b67f95c-27a5-42dd-8c95-efe452094dc3@gmail.com>
Date: Tue, 21 Apr 2026 10:54:21 +0200
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Subject: Re: A very quick observation of dangling pointers in Postgres
 pathlists
To: David Rowley <dgrowleyml@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
 Tom Lane <tgl@sss.pgh.pa.us>, "a.rybakina" <a.rybakina@postgrespro.ru>
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 <CAApHDvpBZ_hj0p1fYYQCxk9eCfNSsjFfR+LYCEryek9Mxh1V0Q@mail.gmail.com>
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From: Andrei Lepikhov <lepihov@gmail.com>
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On 21/04/2026 10:35, David Rowley wrote:
> On Tue, 21 Apr 2026 at 19:29, Andrei Lepikhov <lepihov@gmail.com> wrote:
>> I've attached a patch that shows how to fix the issue. Some regression tests
>> change because of a hidden rule where a projection and its subpath have
>> different target lists. Right now, the patch always enforces a projection, even
>> if the target lists are the same. This is still open for discussion on whether
>> there's a better way to handle it.
> 
> IMO, we should write a function like copy_path() or reparent_path(),
> which creates a copy of the given Path, or the latter also would copy
> then set the ->parent to the given RelOptInfo.  Any time we use a path
> directly from the pathlist of another RelOptInfo, we should reparent
> or copy it. We could add an Assert in add_path() to check the new path
> has the correct parent to help us find the places where we forget to
> do this.

It would be great to have a copy_path() function. At the moment, I create a
limited version each time in an extension module, using
reparameterize_path_by_child as a guide since it ensures the core can handle
path copies.
Do you mean we can introduce such a copy routine to fix current issue? Here is
the problem: dangling pointers are detected only by external tools. I can't
imagine an SQL reproducer to test this machinery.

-- 
regards, Andrei Lepikhov,
pgEdge