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Subject: Re: Fix SET EXPRESSION for virtual columns with whole-row
 dependencies
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: <5B739323-C897-4B1A-9BFC-656E5CB4AD58@gmail.com>
Date: Wed, 10 Jun 2026 17:05:30 +0800
Cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>,
 Peter Eisentraut <peter@eisentraut.org>
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To: jian he <jian.universality@gmail.com>
Archived-At: 
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> On Jun 9, 2026, at 15:28, Chao Li <li.evan.chao@gmail.com> wrote:
>=20
>=20
>=20
>> On Jun 9, 2026, at 15:23, jian he <jian.universality@gmail.com> =
wrote:
>>=20
>> On Tue, Jun 9, 2026 at 2:23=E2=80=AFPM Chao Li =
<li.evan.chao@gmail.com> wrote:
>>>=20
>>> Hi,
>>>=20
>>> While testing =E2=80=9C[f80bedd52] Allow ALTER COLUMN SET EXPRESSION =
on virtual columns=E2=80=9D, I found that the feature missed handling =
whole-row check constraints.
>>>=20
>>> Here is a repro:
>>> ```
>>> evantest=3D# create table t(
>>> evantest(#   a int,
>>> evantest(#   b int generated always as (a*2) virtual,
>>> evantest(#   constraint row_c check (t is not null)
>>> evantest(# );
>>> CREATE TABLE
>>> evantest=3D# insert into t(a) values(1);
>>> INSERT 0 1
>>> evantest=3D# alter table t alter b set expression as (nullif(a, 1));
>>> ALTER TABLE
>>> evantest=3D# select * from t;
>>> a | b
>>> ---+---
>>> 1 |
>>> (1 row)
>>> ```
>>>=20
>>> The ALTER TABLE should fail, because it makes b become NULL, which =
breaks the constraint row_c.
>>>=20
>>=20
>> Hi.
>>=20
>> In case you are wondering whole-row objects:
>> ALTER TABLE DROP COLUMN and ALTER COLUMN SET DATA TYPE are covered by
>> https://commitfest.postgresql.org/patch/5988 and
>> https://commitfest.postgresql.org/patch/6055.
>>=20
>> Meanwhile, ALTER TABLE SET EXPRESSION is being handled over in
>> https://commitfest.postgresql.org/patch/6755
>=20
> Thanks for pointing out that. I will review 6755.
>=20
> Best regards,
> --
> Chao Li (Evan)
> HighGo Software Co., Ltd.
> https://www.highgo.com/
>=20

After reading the other implementation in [1], I realized that I had =
missed the partial-index case, so I added coverage for that.

I am still sending an updated version of this patch because my =
implementation is different from the one in [1]. I would like people to =
compare the two approaches and decide which direction is better.

[1] =
https://postgr.es/m/CAJTYsWXOkyeDVbzymWc9sKrq7Y_MUv6XJXN4H9GfsBOPd3NJ+w@ma=
il.gmail.com

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