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From: Richard Guo <guofenglinux@gmail.com>
Date: Mon, 20 Apr 2026 14:25:43 +0900
Message-ID: 
 <CAMbWs48s9EVTz__xq9s0nAGeh-Z918t4VoGD+pxCXOq0mnc_7Q@mail.gmail.com>
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 Fri, Apr 17, 2026 at 7:35=E2=80=AFPM Dean Rasheed <dean.a.rasheed@gmail.=
com> wrote:
> I don't quite buy the argument that the rule action is typically
> small. We have no idea how big it might be. Note that, at that point
> in the code, sub_action is the combination of both the original query
> and the rule action.
>
> I do accept though that rules are not widely used, and that it's not
> worth optimising too much, if it means a lot of extra complexity.
> However, IMO, it is slightly simpler and neater to put the expanded
> generated columns in the replacement list used by
> ReplaceVarsFromTargetList() on sub_action.

Fair point.

> In the attached v2 patch, I've done that by refactoring
> expand_generated_columns_internal(), renaming it to
> get_generated_columns(), and making it just return the list of
> generated column expressions, rather than doing the rewrite -- I never
> particularly liked the separation of concerns between
> expand_generated_columns_internal() and
> expand_generated_columns_in_expr(), especially after the rest of the
> code expanding virtual generated columns was moved out of the
> rewriter, so that expand_generated_columns_in_expr() became the only
> caller of expand_generated_columns_internal(). Doing this simplifies
> the function, since it's no longer necessary to pass it node, rte, and
> result_relation.
>
> With that change, all rewriteRuleAction() needs to do is get the
> generated columns, rewrite any new.attribute references in them, and
> then use that list plus the original target list as the replacement
> list when rewriting sub_action.

Yeah, this is a better approach.  The change looks good to me.

A nitpick: For the comment "The generated column expressions typically
refer to new.attribute ...", maybe we can remove "typically", as
generation expressions always refer to columns of the same relation.

- Richard