Re: pg_plan_advice: FOREIGN_JOIN advice generated for a single-relation foreign scan is not round-trip safe

On Tue, Jun 30, 2026 at 10:52 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Jun 30, 2026 at 10:36 AM Mahendra Singh Thalor
> <mahi6run(at)gmail(dot)com> wrote:
> > While testing pg_plan_advice together with postgres_fdw, I noticed that the
> > advice generated for a single-relation foreign scan is not round-trip safe,
> > which contradicts the "round-trip safe" guarantee documented in
> > contrib/pg_plan_advice/README.
> >
> > When postgres_fdw pushes an aggregate down over a single foreign table, the
> > resulting ForeignScan has scanrelid == 0 but fs_relids names exactly one
> > relation. pg_plan_advice generates FOREIGN_JOIN(<rel>) advice for it. However,
> > the advice parser requires a FOREIGN_JOIN target to name more than one
> > relation, so feeding the generated advice back in fails to parse.
>
> Thanks for the report. I think this patch looks correct, but the
> comments are a bit overly verbose. Barring objections or other
> comments, I'll tighten this up, commit and back-patch.

Done. I ended up making some changes to the test case as well,
basically to make it less likely that it could accidentally pass.

--
Robert Haas
EDB: http://www.enterprisedb.com