Re: crashes due to setting max_parallel_workers=0

On 03/27/2017 05:51 PM, Robert Haas wrote:
> On Mon, Mar 27, 2017 at 9:54 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> On Mon, Mar 27, 2017 at 1:29 AM, Rushabh Lathia
>>> <rushabh(dot)lathia(at)gmail(dot)com> wrote:
>>>> But it seems a bit futile to produce the parallel plan in the first place,
>>>> because with max_parallel_workers=0 we can't possibly get any parallel
>>>> workers ever. I wonder why compute_parallel_worker() only looks at
>>>> max_parallel_workers_per_gather, i.e. why shouldn't it do:
>>>> parallel_workers = Min(parallel_workers, max_parallel_workers);
>>>> Perhaps this was discussed and is actually intentional, though.
>>
>>> It was intentional. See the last paragraph of
>>> https://www.postgresql.org/message-id/CA%2BTgmoaMSn6a1780VutfsarCu0LCr%3DCO2yi4vLUo-JQbn4YuRA@mail.gmail.com
>>
>> Since this has now come up twice, I suggest adding a comment there
>> that explains why we're intentionally ignoring max_parallel_workers.
>
> Hey, imagine if the comments explained the logic behind the code!
>
> Good idea. How about the attached?
>

Certainly an improvement. But perhaps we should also mention this at
compute_parallel_worker, i.e. that not looking at max_parallel_workers
is intentional.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services