Re: max_parallel_degree > 0 for 9.6 beta

On Thu, Aug 4, 2016 at 12:56 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Noah Misch <noah(at)leadboat(dot)com> writes:
>> On Wed, Apr 20, 2016 at 02:28:15PM -0400, Tom Lane wrote:
>>> +1, but let's put an entry on the 9.6 open-items page to remind us to
>>> make that decision at the right time.
>
>> It's that time. Do we restore the max_parallel_workers_per_gather=0 default,
>> or is enabling this by default the right thing after all?
>
> At this point I'd have to vote against enabling by default in 9.6. The
> fact that in the past week we've found bugs as bad as e1a93dd6a does not
> give me a warm fuzzy feeling about the parallel-query code being ready
> for prime time.
>
> Of course the question is how do we ever get to that point if we chicken
> out with enabling it by default now. Maybe we could keep it turned on
> in HEAD.

What do other people think about this topic?

Personally, I've found the process of fixing the parallel query bugs
rather exhausting, and if we leave it turned on by default in 9.6,
we'll probably get a lot more of those bug reports a lot sooner than
we will otherwise. But I'd kind of rather get it out of the way than
put it off: those bugs need to be fixed at some point, and we won't
find them if nobody runs the code. However, we have a long track
record of being cautious about things like this, so I would be OK with
the idea of disabling this in the 9.6 branch and leaving it turned on
in master, with the hope of shipping it enabled-by-default in v10. I
think it would not be good to leave disabled it by default in master
-- the code won't get much testing then, we won't find the bugs, and
we'll build more stuff on top of what we've already got without
finding the cracks in the foundation. I think users will be more
inclined to forgive us for parallel query bugs in 2016 than in 2026;
better to debug it before the honeymoon wears off.

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