Re: Rename max_parallel_degree?

On Tue, May 31, 2016 at 4:04 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
>> Robert Haas wrote:
>>> So I think in the long run we should have three limits:
>>>
>>> 1. Cluster-wide limit on number of worker processes for all purposes
>>> (currently, max_worker_processes).
>>>
>>> 2. Cluster-wide limit on number of worker processes for parallelism
>>> (don't have this yet).
>>>
>>> 3. Per-operation limit on number of worker processes for parallelism
>>> (currently, max_parallel_degree).
>>>
>>> Whatever we rename, there needs to be enough semantic space between #1
>>> and #3 to allow for the possibility - I think the very likely
>>> possibility - that we will eventually also want #2.
>
>> max_background_workers sounds fine to me for #1, and I propose to add #2
>> in 9.6 rather than wait.
>
> +1 to both points.
>
>> max_total_parallel_query_workers ?
>
> The name should be closely related to what we use for #3.  I could go for
> max_total_parallel_workers for #2 and max_parallel_workers for #3.
> Or maybe max_parallel_workers_total?

I just want to point out that if we change #1, we're breaking
postgresql.conf compatibility for, IMHO, not a whole lot of benefit.
I'd just leave it alone.

I would propose to call #2 max_parallel_workers and #3
max_parallel_degree, which I think is clear as daylight, but since I
invented all of these names, I guess I'm biased.

In terms of forward-compatibility, one thing to note is that we
currently have only parallel QUERY, but in the future we will no doubt
also have parallel operations that are not queries, like VACUUM and
CLUSTER and ALTER TABLE.  If we put the word "query" into the name of
a GUC, we're committing to the idea that it only applies to queries,
and that there will be some other limit for non-queries.  If we don't
put the word query in there, we are not necessarily committing to the
reverse, but we're at least leaning in that direction.  So far I've
largely dodged having to make a decision about this, because talking
about the degree of parallelism for CLUSTER makes just as much sense
as talking about the degree of parallelism for a query, but we could
also decide to have e.g. maintenance_parallel_degree instead of
max_parallel_degree for such operations.  But as we commit to names
it's something to be aware of.

-- 
Robert Haas
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