Re: Parallel Index Scans

On Fri, Oct 21, 2016 at 10:55 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Oct 21, 2016 at 9:27 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> I think the parallel_workers reloption should override the degree of
>>> parallelism for any sort of parallel scan on that table. Had I
>>> intended it to apply only to sequential scans, I would have named it
>>> differently.
>>
>> I think there is big difference of size of relation to scan between
>> parallel sequential scan and parallel (range) index scan which could
>> make it difficult for user to choose the value of this parameter. Why
>> do you think that the parallel_workers reloption should suffice all
>> type of scans for a table? I could only think of providing it based
>> on thinking that lesser config knobs makes life easier.
>
> Well, we could do that, but it would be fairly complicated and it
> doesn't seem to me to be the right place to focus our efforts. I'd
> rather try to figure out some way to make the planner smarter, because
> even if users can override the number of workers on a
> per-table-per-scan-type basis, they're probably still going to find
> using parallel query pretty frustrating unless we make the
> number-of-workers formula smarter than it is today. Anyway, even if
> we do decide to add more reloptions than just parallel_degree someday,
> couldn't that be left for a separate patch?
>

That makes sense to me. As of now, patch doesn't consider reloptions
for parallel index scans. So, I think we can leave it as it is and
then later as a separate patch decide whether to use reloption of
table or a separate reloption for index would be better choice.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com