Re: Parallelize correlated subqueries that execute within each worker

On Tue, Jan 30, 2024 at 10:34 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> James Coleman <jtc331(at)gmail(dot)com> writes:
> > I've finally had a chance to look at this, and I don't believe there's
> > any real failure here, merely drift of how the planner works on master
> > resulting in this query now being eligible for a different plan shape.
>
> > I was a bit wary at first because the changing test query is one I'd
> > previously referenced in [1] as likely exposing the bug I'd fixed
> > where params where being used across worker boundaries. However
> > looking at the diff in the patch at that point (v10) that particular
> > test query formed a different plan shape (there were two gather nodes
> > being created, and params crossing between them).
>
> > But in the current revision of master with the current patch applied
> > that's no longer true: we have a Gather node, and the Subplan using
> > the param is properly under that Gather node, and the param should be
> > being both generated and consumed within the same worker process.
>
> Hmm ... so the question this raises for me is: was that test intended
> to verify behavior of params being passed across workers? If so,
> haven't you broken the point of the test? This doesn't mean that
> your code change is wrong; but I think maybe you need to find a way
> to modify that test case so that it still tests what it's meant to.
> This is a common hazard when changing the planner's behavior.

I'd been thinking it was covered by another test I'd added in 0001,
but looking at it again that test doesn't exercise parallel append
(though it does exercise a different case of cross-worker param
usage), so I'll add another test for the parallel append behavior.

Regards,
James Coleman