Re: Choosing parallel_degree

On 16 March 2016 at 13:26, Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com> wrote:
> On 15/03/2016 21:12, Robert Haas wrote:
>> I thought about this a bit more. There are a couple of easy things we
>> could do here.
>>
>> The 1000-page threshold could be made into a GUC.
>>
>> We could add a per-table reloption for parallel-degree that would
>> override the calculation.
>>
>> Neither of those things is very smart, but they'd probably both help
>> some people. If someone is able to produce a patch for either or both
>> of these things *quickly*, we could possibly try to squeeze it into
>> 9.6 as a cleanup of work already done.
>>
>
> I'm not too familiar with parallel planning, but I tried to implement
> both in attached patch. I didn't put much effort into the
> parallel_threshold GUC documentation, because I didn't really see a good
> way to explain it. I'd e happy to improve it if needed. Also, to make
> this parameter easier to tune for users, perhaps we could divide the
> default value by 3 and use it as is in the first iteration in
> create_parallel_path() ?
>
> Also, global max_parallel_degree still needs to be at least 1 for the
> per table value to be considered.

Thanks for working on this. I've only skimmed the patch so far, but
will try to look more closely later.

This did get me wondering why we have the parallel_threshold at all,
and not just allow the parallel_setup_cost to make parallel plans look
less favourable for smaller relations. I assume that this is so that
we don't burden the planner with the overhead of generating parallel
paths for smaller relations?

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David Rowley http://www.2ndQuadrant.com/
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