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From: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Date: Mon, 13 Apr 2026 00:27:44 -0700
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 <CAHg+QDf1Y=dZR8i+q3nnjEt2qeJpDG_pvrWtozXTrdu3J8H-FQ@mail.gmail.com>
Subject: Re: Bug: Rule actions see wrong values for generated columns (NEW.gen
 reads OLD value)
To: Chao Li <li.evan.chao@gmail.com>
Cc: Richard Guo <guofenglinux@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Peter Eisentraut <peter@eisentraut.org>
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Hi Chao,

On Mon, Apr 13, 2026 at 12:21=E2=80=AFAM Chao Li <li.evan.chao@gmail.com> w=
rote:

>
>
> > On Apr 13, 2026, at 10:35, Richard Guo <guofenglinux@gmail.com> wrote:
> >
> > 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.
> >
> > 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.
> >
> > cc-ing Peter.
> >
> > - Richard
> >
>
> Hi Richard and Satya,
>
> I reproduced the bug following Satya=E2=80=99s procedure and spent some t=
ime
> 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 missin=
g
> 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 column=
s
> 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 generate=
d
> 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=
.


I have a patch with me, let me post it shortly.

Thanks,
Satya

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<div dir=3D"auto">Hi Chao,</div><div><br><div class=3D"gmail_quote gmail_qu=
ote_container"><div dir=3D"ltr" class=3D"gmail_attr">On Mon, Apr 13, 2026 a=
t 12:21=E2=80=AFAM Chao Li &lt;<a href=3D"mailto:li.evan.chao@gmail.com">li=
.evan.chao@gmail.com</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quo=
te" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204=
);padding-left:1ex"><br>
<br>
&gt; On Apr 13, 2026, at 10:35, Richard Guo &lt;<a href=3D"mailto:guofengli=
nux@gmail.com" target=3D"_blank">guofenglinux@gmail.com</a>&gt; wrote:<br>
&gt; <br>
&gt; On Mon, Apr 13, 2026 at 10:59=E2=80=AFAM SATYANARAYANA NARLAPURAM<br>
&gt; &lt;<a href=3D"mailto:satyanarlapuram@gmail.com" target=3D"_blank">sat=
yanarlapuram@gmail.com</a>&gt; wrote:<br>
&gt;&gt; NEW.&lt;generated_coulmn&gt; is resolved to the OLD row&#39;s valu=
e<br>
&gt;&gt; for update or NULL for insert cases in a DO ALSO rule action for<b=
r>
&gt;&gt; generated columns.=C2=A0 This bug affects both stored and virtual<=
br>
&gt;&gt; generated columns. Reporting here to see if this is a known issue<=
br>
&gt;&gt; with generated columns.<br>
&gt; <br>
&gt; I didn&#39;t find related item in open items.=C2=A0 This does not seem=
 to be a<br>
&gt; known issue.=C2=A0 I think we should fix it anyway.<br>
&gt; <br>
&gt; cc-ing Peter.<br>
&gt; <br>
&gt; - Richard<br>
&gt; <br>
<br>
Hi Richard and Satya,<br>
<br>
I reproduced the bug following Satya=E2=80=99s procedure and spent some tim=
e debugging it.<br>
<br>
I think the issue is that rewriteTargetListIU() removes generated columns f=
rom the target list, as described by this comment:<br>
```<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 if (att_tup-&gt;attgenerated)<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 {<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 /*<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0* virtual generated column =
stores a null value; stored generated<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0* column will be fixed in e=
xecutor<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0*/<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 new_tle =3D NULL;<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 }<br>
```<br>
<br>
Later, when the rule action is rewritten, ReplaceVarsFromTargetList() canno=
t find a target list entry for NEW.gen. For UPDATE rules, the missing NEW c=
olumn is handled with REPLACEVARS_CHANGE_VARNO, so it falls back to referen=
cing the original target relation row, which gives the old value.<br>
<br>
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 q=
uick and dirty code and it seems to fix both stored and virtual generated c=
olumns for me.<br>
<br>
Do either of you plan to propose a patch for this? If so, please go ahead a=
nd I can review it. Otherwise, I can propose a patch in a couple of days.</=
blockquote><div dir=3D"auto"><br></div><div dir=3D"auto">I have a patch wit=
h me, let me post it shortly.</div><div dir=3D"auto"><br></div><div dir=3D"=
auto">Thanks,</div><div dir=3D"auto">Satya</div></div></div>

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