Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

On Sat, Jan 20, 2018 at 2:57 AM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> On Sat, Jan 20, 2018 at 6:32 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Fri, Jan 19, 2018 at 12:16 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>>> Clarity on what I should do about parallel_leader_participation in the
>>> next revision would be useful at this point. You seem to either want
>>> me to remove it from consideration entirely, or to remove the code
>>> that specifically disallows a "degenerate parallel CREATE INDEX". I
>>> need a final answer on that.
>>
>> Right. I do think that we should do one of those things, and I lean
>> towards removing it entirely, but I'm not entirely sure. Rather
>> than making an executive decision immediately, I'd like to wait a few
>> days to give others a chance to comment. I am hoping that we might get
>> some other opinions, especially from Thomas who implemented
>> parallel_leader_participation, or maybe Amit who has been reviewing
>> recently, or anyone else who is paying attention to this thread.
>
> Well, I see parallel_leader_participation as having these reasons to exist:
>
> 1. Gather could in rare circumstances not run the plan in the leader.
> This can hide bugs. It's good to be able to force that behaviour for
> testing.
>

Or reverse is also possible which means the workers won't get chance
to run the plan in which case we can use parallel_leader_participation
= off to test workers behavior. As said before, I see only that as
the reason to keep parallel_leader_participation in this patch. If we
decide to do that way, then I think we should remove the code that
specifically disallows a "degenerate parallel CREATE INDEX" as that
seems to be confusing. If we go this way, then I think we should use
the wording suggested by Robert in one of its email [1] to describe
the usage of parallel_leader_participation.

BTW, is there any other way for "parallel create index" to force that
the work is done by workers? I am insisting on having something which
can test the code path in workers because we have found quite a few
bugs using that idea.

[1] - https://www.postgresql.org/message-id/CA%2BTgmoYN-YQU9JsGQcqFLovZ-C%2BXgp1_xhJQad%3DcunGG-_p5gg%40mail.gmail.com

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