On Sun, May 29, 2016 at 1:33 AM, Noah Misch <n...@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;
rhaas=# alter table pgbench_accounts set (parallel_degree = 10);
rhaas=# explain (analyze) select count(*) from pgbench_accounts;

 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
         ->  Partial Aggregate  (cost=176435.00..176435.01 rows=1
width=8) (actual time=375.569..375.569 rows=1 loops=9)
               ->  Parallel Seq Scan on pgbench_accounts
(cost=0.00..173935.00 rows=1000000 width=0) (actual
time=0.103..260.352 rows=1111111 loops=9)
 Planning time: 0.135 ms
 Execution time: 384.387 ms
(8 rows)

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment: parallel-degree-1based.patch
Description: binary/octet-stream

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