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

On Fri, Apr 17, 2026 at 7:35 PM Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)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