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From: Richard Guo <guofenglinux@gmail.com>
Date: Tue, 14 Apr 2026 12:27:39 +0900
Message-ID: 
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Subject: Re: Bug: Rule actions see wrong values for generated columns (NEW.gen
 reads OLD value)
To: Dean Rasheed <dean.a.rasheed@gmail.com>
Cc: Chao Li <li.evan.chao@gmail.com>,
	SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Peter Eisentraut <peter@eisentraut.org>
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On Mon, Apr 13, 2026 at 8:04=E2=80=AFPM Dean Rasheed <dean.a.rasheed@gmail.=
com> wrote:
> On Mon, 13 Apr 2026, 09:20 Richard Guo, <guofenglinux@gmail.com> wrote:
>> I think a simpler fix might be to expand generated column references
>> in the NEW relation to their generation expressions before
>> ReplaceVarsFromTargetList resolves NEW references, so that the base
>> column Vars within the expressions can be correctly resolved.
>> Something like attached.

> One thing about that approach is that it leads to 2 full rewrites of the =
rule action using ReplaceVarsFromTargetList(). I think that could be avoide=
d by using including generated column expressions in the targetlist passed =
to ReplaceVarsFromTargetList() by rewriteRuleAction(). I haven't tried it, =
but I imagine it could reuse some code from expand_generated_columns_intern=
al().

I considered it, but I'm afraid it doesn't work directly, because
replace_rte_variables_mutator returns the callback's replacement node
without recursing into its children.

Take Satya's repro as an example.  If we add the generation expression
for gen to the UPDATE's targetlist, the list would be:

    TargetEntry 1: resno=3D2, expr=3DConst(100)       -- a =3D 100
    TargetEntry 2: resno=3D3, expr=3DVar(3, 2) * 2    -- gen =3D NEW.a * 2

When ReplaceVarsFromTargetList processes Var(3, 3) (NEW.gen) in the
rule action, it finds resno=3D3 and substitutes Var(3, 2) * 2.  However,
replace_rte_variables_mutator returns this replacement directly to its
caller; it does not recurse into the replacement's children to look
for further matching Vars.  So the inner Var(3, 2) (NEW.a) is never
processed, even though resno=3D2 with Const(100) is right there in the
targetlist.  The Var(3, 2) survives into the planner and would cause
problems.

It could be made to work by pre-resolving the generation expressions'
base column Vars before adding them to the UPDATE's targetlist.  For
each generated column, we'd call ReplaceVarsFromTargetList on the
generation expression to resolve its base column Vars, then add the
fully resolved expression to the targetlist.  But this seems to add
code complexity.  And I'm not sure about the performance difference
between these two approaches.  I expect that rule action trees are
typically small.

- Richard