From: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Guo <guofenglinux@gmail.com>
cc: Sergey Shinderuk <s.shinderuk@postgrespro.ru>,
        pgsql-bugs@lists.postgresql.org, David Rowley <dgrowleyml@gmail.com>
Subject: Re: BUG #19412: Wrong query result with not null constraint
In-reply-to: <CAMbWs481y0EmS7fFJE9DEjjHGJO4mh9nPmusPDQ77Psxgo96ww@mail.gmail.com>
References: <19412-1d0318089b86859e@postgresql.org> <CAApHDvpXarz-D6tP-7y4ZrtnhXtdbRY0kQxRDQTVc3Z9DckYVw@mail.gmail.com> <CAMbWs49kVgb_wS4VFuQpfZ+Re4tWVn7vmheR6HqKppnNuEaH-g@mail.gmail.com> <CAMbWs49NVUNpQPwGb56SCSQL-bkjtxnkCb6Ot7dBxGndJoA3Pw@mail.gmail.com> <60be6cb1-10de-4e24-82ce-b02650442d97@postgrespro.ru> <CAMbWs481y0EmS7fFJE9DEjjHGJO4mh9nPmusPDQ77Psxgo96ww@mail.gmail.com>
Comments: In-reply-to Richard Guo <guofenglinux@gmail.com>
	message dated "Thu, 19 Feb 2026 10:49:44 +0900"
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Date: Thu, 19 Feb 2026 13:25:48 -0500
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Richard Guo <guofenglinux@gmail.com> writes:
> On Wed, Feb 18, 2026 at 9:03 PM Sergey Shinderuk
> <s.shinderuk@postgrespro.ru> wrote:
>> I'm not familiar with the code, just curios. There is a long comment
>> above saying "You might think we need to adjust var->varnullingrels, but
>> that shouldn't need any changes." Doesn't it need an update?

> No, I don't think we need to update it.  That comment explains why
> varnullingrels do not require translation (since they are outer join
> relids, not baserel relids).  It's unrelated to what this patch does,
> which is about propagating varnullingrels into the translated Var.

I agree with this fix: I think the code is like it is simply because
it didn't occur to me that the child Vars could have any nullingrel
bits yet.  However, I don't agree that that comment needs no updates.
I suggest something like

-         * Below, we just propagate var->varnullingrels into the translated
-         * Var.
+         * Below, we just merge var->varnullingrels into the translated
+         * Var.  (We must merge not just copy: the child Var could have
+         * some nullingrel bits set already, and we mustn't drop those.)

Also, I think I'd then drop the comment you added adjacent to the
actual update; it seems redundant if the earlier comment says this.

I agree with back-patching to v16.  This particular example doesn't
misbehave in versions that don't have the drop-allegedly-redundant-
NOT-NULL-tests logic, but the varnullingrels are certainly wrong
all the way back, so possibly there are other examples that do
misbehave in v16.

			regards, tom lane