Re: Rename max_parallel_degree? - Mailing list pgsql-hackers

On 05/31/2016 09:15 AM, Robert Haas wrote:
> On Sun, May 29, 2016 at 1:33 AM, Noah Misch <noah@leadboat.com> wrote:
>> On Fri, May 06, 2016 at 02:52:30PM -0400, Robert Haas wrote:
>>> OK, my reading of this thread is that there is a consensus is to
>>> redefine max_parallel_degree=1 as "no parallelism" and
>>> max_parallel_degree>1 as "parallelism using a leader plus N-1
>>> workers", and along with that, to keep the names unchanged.  However,
>>> I don't think I can get that done before beta1, at least not without a
>>> serious risk of breaking stuff.  I can look at this post-beta1.
>>
>> [This is a generic notification.]
>>
>> The above-described topic is currently a PostgreSQL 9.6 open item.  Robert,
>> since you committed the patch believed to have created it, you own this open
>> item.  If some other commit is more relevant or if this does not belong as a
>> 9.6 open item, please let us know.  Otherwise, please observe the policy on
>> open item ownership[1] and send a status update within 72 hours of this
>> message.  Include a date for your subsequent status update.  Testers may
>> discover new open items at any time, and I want to plan to get them all fixed
>> well in advance of shipping 9.6rc1.  Consequently, I will appreciate your
>> efforts toward speedy resolution.  Thanks.
> 
> Here is a patch.  Note that I still don't agree with this change, but
> I'm bowing to the will of the group.
> 
> I think that some of the people who were in favor of this change
> should review this patch, including especially the language I wrote
> for the documentation.  If that happens, and the reviews are positive,
> then I will commit this.  If that does not happen, then I will
> interpret that to mean that there isn't actually all that much
> interest in changing this after all and will accordingly recommend
> that this open item be removed without further action.
> 
> Here is a test which shows how it works:
> 
> rhaas=# set max_parallel_degree = 100;
> SET
> rhaas=# alter table pgbench_accounts set (parallel_degree = 10);
> ALTER TABLE
> rhaas=# explain (analyze) select count(*) from pgbench_accounts;
> 
> QUERY PLAN
>
--------------------------------------------------------------------------------------------------------------------------------------------------------
>  Finalize Aggregate  (cost=177436.04..177436.05 rows=1 width=8)
> (actual time=383.244..383.244 rows=1 loops=1)
>    ->  Gather  (cost=177435.00..177436.01 rows=10 width=8) (actual
> time=383.040..383.237 rows=9 loops=1)
>          Workers Planned: 9
>          Workers Launched: 8


I realize there's a lot of water under the bridge here, but I think
we're going to get 1000 questions on -general of the type:  "I asked for
8 parallel workers, why did I only get 7?".  I believe we will regret
this change.

So, one vote from me to revert.

-- 
--
Josh Berkus
Red Hat OSAS
(any opinions are my own)