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From: Richard Guo <[email protected]>
To: Alexander Lakhin <[email protected]>
Cc: Tender Wang <[email protected]>
Cc: Tomasz Rybak <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: David Rowley <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Should consider materializing the cheapest inner path in consider_parallel_nestloop()
Date: Mon, 22 Jul 2024 10:43:50 +0800
Message-ID: <CAMbWs4_VNQNV-vSKWKDC7J-ts1qqy_TfZ3wJyA-bzS_d_ioxjw@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
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	<[email protected]>

On Fri, Jul 19, 2024 at 12:00 PM Alexander Lakhin <[email protected]> wrote:
> 18.07.2024 17:30, Richard Guo wrote:
> > I have no idea why the underlying statistics changed, but it seems
> > that this slight change is sufficent to result in a different plan.
>
> I think it could be caused by the same reason as [1] and I really can
> easily (without multiple instances/loops. just with `make check`) reproduce
> the failure with cranky-ConditionalLockBufferForCleanup.patch (but
> targeted for "VACUUM ANALYZE tenk1;").

Yeah.  Anyway I think we need to make the test more tolerant of slight
variations in the statistics.

> > According to the discussion in [1], I think what we wanted to test
> > with this query is that parallel nestloop join is not generated if the
> > inner path is not parallel-safe.  Therefore, I modified this test case
> > to use a lateral join, rendering the inner path not parallel-safe
> > while also enforcing the join order.  Please see attached.
>
> The modified test survives my testing procedure. Thank you for the patch!

Thanks for testing this patch.  I've pushed it.

Thanks
Richard

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