Re: A gap in preventing bogus parameterizations in a798660ebe3

On Mon, Nov 9, 2026 at 4:47 AM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
> I'm now fairly confident the fix is correct. I'd still welcome someone
> else doing a quick review, of course.

I looked at the master patch, and it LGTM.

One nit on the commit message: "we still reject the path, just a bit
later" isn't quite accurate. AFAICS nothing rejects such a path
later. It'd be added to the joinrel's pathlist, just that it can
never be used, because its parameterization includes a relid within
the joinrel, which the other input of an upper join can never supply.
Maybe something like "In non-assert builds this is mostly harmless,
since such paths can never appear in a complete plan."

This check only covers parameters that need an outer join completed at
the current join, though. It misses the case where a parameter can
only be supplied above some outer join that itself has to be formed
above the current join. If such a parameter comes from a lateral
reference, every path of the joinrel is dead, and the join has to be
rejected in join_is_legal() instead. I'm working on that in [1]. So
if your join generator hits "failed to build any N-way joins" with
LATERAL, it's likely that issue rather than another hole in this
patch.

[1] https://postgr.es/m/CAMbWs48YGwbxxz-3_g0snstuVNWdkMfJwM7iwqf8NY5_uLQ23Q@mail.gmail.com

- Richard