From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Oddity with parallel safety test for scan/join target in grouping_planner
Date: Mon, 11 Mar 2019 14:08:51 +0900
Message-ID: <5C85ED63.8000805@lab.ntt.co.jp> (raw)
In-Reply-To: <32401.1552277190@sss.pgh.pa.us>
References: <5C75303E.8020303@lab.ntt.co.jp>
<CA+TgmobmV=Zw2OEJKUorJe=e+_fCzsvYErR6tXxTjCzK45x9RQ@mail.gmail.com>
<5C7908B9.5070407@lab.ntt.co.jp>
<24818.1552077419@sss.pgh.pa.us>
<5C85DC65.2050409@lab.ntt.co.jp>
<32401.1552277190@sss.pgh.pa.us>
(2019/03/11 13:06), Tom Lane wrote:
> Etsuro Fujita<fujita.etsuro@lab.ntt.co.jp> writes:
>>>> The parallel safety of the final scan/join target is determined from the
>>>> grouping target, not that target, which [ is wrong ]
>
>> This would only affect plan quality a little bit, so I don't think we
>> need a regression test for this, either, but the fix might destabilize
>> existing plan choices, so we should apply it to HEAD only?
>
> Is that the only possible outcome? Per Robert's summary quoted above,
> it seems like it might be possible for the code to decide that the final
> scan/join target to be parallel safe when it is not, leading to outright
> wrong answers or query failures.
Maybe I'm missing something, but I think that if the final scan/join
target is not parallel-safe, then the grouping target would not be
parallel-safe either, by the construction of the two targets, so I don't
think that we have such a correctness issue.
Best regards,
Etsuro Fujita
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To: pgsql-hackers@postgresql.org
Cc: fujita.etsuro@lab.ntt.co.jp, tgl@sss.pgh.pa.us, robertmhaas@gmail.com
Subject: Re: Oddity with parallel safety test for scan/join target in grouping_planner
In-Reply-To: <5C85ED63.8000805@lab.ntt.co.jp>
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