Re: initial pruning in parallel append - Mailing list pgsql-hackers

On Wed, Aug 9, 2023 at 9:48 PM Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Aug 9, 2023 at 6:22 AM Amit Langote <amitlangote09@gmail.com> wrote:
> > > > I'm assuming it's not
> > > > too ugly if ExecInitAppend() uses IsParallelWorker() to decide whether
> > > > it should be writing to EState.es_part_prune_results or reading from
> > > > it -- the former if in the leader and the latter in a worker.
> > >
> > > I don't think that's too ugly. I mean you have to have an if statement
> > > someplace.
> >
> > Yes, that makes sense.
> >
> > It's just that I thought maybe I haven't thought hard enough about
> > options before adding a new IsParallelWorker(), because I don't find
> > too many instances of IsParallelWorker() in the generic executor code.
> > I think that's because most parallel worker-specific logic lives in
> > execParallel.c or in Exec*Worker() functions outside that file, so the
> > generic code remains parallel query agnostic as much as possible.
>
> Oh, actually, there is an issue here. IsParallelWorker() is not the
> right test. Imagine that there's a parallel query which launches some
> workers, and one of those calls a user-defined function which again
> uses parallelism, launching more workers. This may not be possible
> today, I don't really remember, but the test should be "am I a
> parallel worker with respect to this plan?" not "am I a parallel
> worker at all?".Not quite sure what the best way to code that is. If
> we could just test whether we have a ParallelWorkerContext, it would
> be easy...

I checked enough to be sure that IsParallelWorker() is reliable at the
time of ExecutorStart() / ExecInitNode() in ParallelQueryMain() in a
worker.   However, ParallelWorkerContext is not available at that
point.  Here's the relevant part of ParallelQueryMain():

    /* Start up the executor */
    queryDesc->plannedstmt->jitFlags = fpes->jit_flags;
    ExecutorStart(queryDesc, fpes->eflags);

    /* Special executor initialization steps for parallel workers */
    queryDesc->planstate->state->es_query_dsa = area;
    if (DsaPointerIsValid(fpes->param_exec))
    {
        char       *paramexec_space;

        paramexec_space = dsa_get_address(area, fpes->param_exec);
        RestoreParamExecParams(paramexec_space, queryDesc->estate);
    }
    pwcxt.toc = toc;
    pwcxt.seg = seg;
    ExecParallelInitializeWorker(queryDesc->planstate, &pwcxt);

BTW, we do also use IsParallelWorker() in ExecGetRangeTableRelation()
which also probably only runs during ExecInitNode(), same as
ExecInitPartitionPruning() that this patch adds it to.

--
Thanks, Amit Langote
EDB: http://www.enterprisedb.com