Re: max_parallel_degree context level

On 11 February 2016 at 12:40, Thom Brown <thom(at)linux(dot)com> wrote:

> Hi all,
>
> As it currently stands, max_parallel_degree is set to a superuser
> context, but we probably want to discuss whether we want to keep it
> this way prior to releasing 9.6. Might we want to reduce its level so
> that users can adjust it accordingly? They'd still be limited by
> max_worker_processes, so they'd at least be constrained by that
> setting.
>

A few questions and thoughts to help decide...

Does it take into account the parallel degree during planning?
Does it take into account the actual parallel degree during planning?

If you make max_worker_processes USERSET won't everybody just set it to
max_worker_processes?

How do you prevent or control that? Is that limited by the user's
connection limit?

How does the server behave when less servers are available than
max_parallel_degree?

Is it slower if you request N workers, yet only 1 is available?

Does pg_stat_activity show the number of parallel workers active for a
controliing process?
Do parallel workers also show in pg_stat_activity at all?
If so, does it show who currently has them?
Does pg_stat_statements record how many workers were available during
execution?

Is there a way to prevent execution if too few parallel workers are
available?

--
Simon Riggs http://www.2ndQuadrant.com/
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