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From: Chao Li <[email protected]>
To: Richard Guo <[email protected]>
Cc: Dean Rasheed <[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: Tue, 14 Apr 2026 14:28:24 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAMbWs49DcHqOtd_BzuFcr9Va+tD-2fNs8bGsbH_Zy2bXUTNM1g@mail.gmail.com>
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	<CAMbWs4-SJ6B26ciFu_K_2M1ObDA=GirV87N1BKeAtSRmQGATUA@mail.gmail.com>
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	<CAMbWs48=QrLy8gZprTfCmEtzoaUku54JXvAcnoRSo+OC8vUpwA@mail.gmail.com>
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> On Apr 14, 2026, at 11:27, Richard Guo <[email protected]> wrote:
> 
> On Mon, Apr 13, 2026 at 8:04 PM Dean Rasheed <[email protected]> wrote:
>> On Mon, 13 Apr 2026, 09:20 Richard Guo, <[email protected]> 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 avoided 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_internal().
> 
> 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=2, expr=Const(100)       -- a = 100
>    TargetEntry 2: resno=3, expr=Var(3, 2) * 2    -- gen = NEW.a * 2
> 
> When ReplaceVarsFromTargetList processes Var(3, 3) (NEW.gen) in the
> rule action, it finds resno=3 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=2 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

My implementation has pre-resolved the generation expressions, that’s why all tests passed. But I agree my change is heavier as I had to add a new static helper function.

If we think rule actions are usually small enough that the extra full-tree pass would not be an issue, then v1 may be preferable for simplicity.

My only comment on v1 is the typo in generated_virtual.sql where “STORED” should be “VIRTUAL”.

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

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