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From: Chao Li <[email protected]>
To: Richard Guo <[email protected]>
Cc: SATYANARAYANA NARLAPURAM <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Subject: Re: Bug: Rule actions see wrong values for generated columns (NEW.gen reads OLD value)
Date: Mon, 13 Apr 2026 15:21:05 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAMbWs4-SJ6B26ciFu_K_2M1ObDA=GirV87N1BKeAtSRmQGATUA@mail.gmail.com>
References: <CAHg+QDexGTmCZzx=73gXkY2ZADS6LRhpnU+-8Y_QmrdTS6yUhA@mail.gmail.com>
	<CAMbWs4-SJ6B26ciFu_K_2M1ObDA=GirV87N1BKeAtSRmQGATUA@mail.gmail.com>



> On Apr 13, 2026, at 10:35, Richard Guo <[email protected]> wrote:
> 
> On Mon, Apr 13, 2026 at 10:59 AM SATYANARAYANA NARLAPURAM
> <[email protected]> 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’s 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 = 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/

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