Re: max_parallel_degree > 0 for 9.6 beta

On Thu, Apr 21, 2016 at 7:20 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Thu, Apr 21, 2016 at 4:01 PM, Gavin Flower
>> <GavinFlower(at)archidevsys(dot)co(dot)nz> wrote:
>>> Why not 4? As most processors now have at least 4 physical cores, & surely
>>> it be more likely to flush out race conditions.
>
>> Because if we did that, then it's extremely likely that people would
>> end up writing queries that are faster only if workers are present,
>> and then not get any workers.
>
> Is that because max_worker_processes is only 8 by default? Maybe we
> need to raise that, at least for beta purposes?

I'm not really in favor of that. I mean, almost all of our default
settings are optimized for running PostgreSQL on, for example, a
Raspberry Pi 2, so it would seem odd to suddenly swing the other
direction and assume that there are more than 8 unused CPU cores. It
doesn't make sense to me to roll out settings in beta that we wouldn't
be willing to release with if they work out. That's why, honestly, I
would prefer max_parallel_degree=1, which I think would be practical
for many real-world deployments. max_parallel_degree=2 is OK. Beyond
that, we're just setting people up to fail, I think. Higher settings
should probably only be used on substantial hardware, and not
everybody has that.

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