Re: Choosing parallel_degree

On 15/03/2016 21:12, Robert Haas wrote:
> On Mon, Mar 14, 2016 at 9:25 PM, David Rowley
> <david.rowley@2ndquadrant.com> wrote:
>> Over in [1] James mentioned about wanting more to be able to have more
>> influence over the partial path's parallel_degree decision.  At risk
>> of a discussion on that hijacking the parallel aggregate thread, I
>> thought I'd start this for anyone who would want to discuss making
>> changes to that.
>>
>> I've attached a simple C program which shows the parallel_degree which
>> will be chosen at the moment. For now it's based on the size of the
>> base relation. Perhaps that will need to be rethought later, perhaps
>> based on costs. But I just don't think it's something for 9.6.
>
> 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.

--
Julien Rouhaud
http://dalibo.com - http://dalibo.org