Re: Assert failure in parallel query due to SubPlan double visit for hash join

Hi Sami,

On Wed, Oct 28, 2026 at 5:17 PM Sami Imseih <samimseih(dot)pg(at)gmail(dot)com> wrote:
> In a non-assert build, this kind of case is definitely not easy to spot,
> unless someone is really scrutinizing these parts of the plan.

Yeah, agreed - its also a somewhat unlikely plan to have, so I don't
think this is seen much in practice, and if it is, people won't
realize its wrong.

> Your patch looks fine to me. I just had a few small comments:

Thanks for reviewing!

> 1/ Maybe rename planstate_tree_collect() to something like
> planstate_tree_collect_distinct(). The important part here is that each
> distinct PlanState appears in the returned list only once, so the name should
> probably say that.

Yeah, good point on naming - if we stay with this structure I'll adjust.

> 2/ Some execParallel.c comments still read as if the functions recurse. For
> example ExecParallelRetrieveInstrumentation(),
> ExecParallelReportInstrumentation(),
> and ExecParallelInitializeWorker() now only deal with a single
> PlanState at a time.

Good point - there are a few that clearly need updating (e.g.
ExecParallelRetrieveInstrumentation jumps out), will revise.

Thanks,
Lukas

--
Lukas Fittl