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Subject: Re: Bug: Rule actions see wrong values for generated columns (NEW.gen
 reads OLD value)
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: 
 <CAMbWs4-SJ6B26ciFu_K_2M1ObDA=GirV87N1BKeAtSRmQGATUA@mail.gmail.com>
Date: Mon, 13 Apr 2026 15:21:05 +0800
Cc: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
 Peter Eisentraut <peter@eisentraut.org>
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To: Richard Guo <guofenglinux@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/22B4A33A-99F3-46F5-BE0C-426A9E1D9ABA%40gmail.com>
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> On Apr 13, 2026, at 10:35, Richard Guo <guofenglinux@gmail.com> wrote:
>=20
> On Mon, Apr 13, 2026 at 10:59=E2=80=AFAM SATYANARAYANA NARLAPURAM
> <satyanarlapuram@gmail.com> wrote:
>> NEW.<generated_coulmn> is resolved to the OLD row's value
>> for update or NULL for insert cases in a DO ALSO rule action for
>> generated columns.  This bug affects both stored and virtual
>> generated columns. Reporting here to see if this is a known issue
>> with generated columns.
>=20
> I didn't find related item in open items.  This does not seem to be a
> known issue.  I think we should fix it anyway.
>=20
> cc-ing Peter.
>=20
> - Richard
>=20

Hi Richard and Satya,

I reproduced the bug following Satya=E2=80=99s procedure and spent some =
time debugging it.

I think the issue is that rewriteTargetListIU() removes generated =
columns from the target list, as described by this comment:
```
        if (att_tup->attgenerated)
        {
            /*
             * virtual generated column stores a null value; stored =
generated
             * column will be fixed in executor
             */
            new_tle =3D NULL;
        }
```

Later, when the rule action is rewritten, ReplaceVarsFromTargetList() =
cannot find a target list entry for NEW.gen. For UPDATE rules, the =
missing NEW column is handled with REPLACEVARS_CHANGE_VARNO, so it falls =
back to referencing the original target relation row, which gives the =
old value.

One possible fix is to build a new target list that adds generated =
columns back when there are rules to fire. I tried the solution locally =
with some quick and dirty code and it seems to fix both stored and =
virtual generated columns for me.

Do either of you plan to propose a patch for this? If so, please go =
ahead and I can review it. Otherwise, I can propose a patch in a couple =
of days.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/