Re: Bug: Rule actions see wrong values for generated columns (NEW.gen reads OLD value)

Hi Chao,

On Mon, Apr 13, 2026 at 12:21 AM Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:

>
>
> > On Apr 13, 2026, at 10:35, Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> >
> > On Mon, Apr 13, 2026 at 10:59 AM SATYANARAYANA NARLAPURAM
> > <satyanarlapuram(at)gmail(dot)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’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.

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

Thanks,
Satya