Re: Disable parallel query by default

On 2025-Jul-14, Tom Lane wrote:

> "Scott Mead" <scott(at)meads(dot)us> writes:
> > I'd like to re-open the discussion for this commitfest item. I
> > still have not been able to find a value for parallel_setup_cost
> > that makes good decisions about parallelism on a user's behalf. I
> > believe that setting the SIGHUP-able max_parallel_workers_per_gather
> > to 0 by default is still the best way to prevent runaway parallel
> > execution behavior.
>
> I still think that proposal has no chance of getting off the ground.
>
> I do agree that the current default cost settings for parallel query
> are over-optimistic and allow us to choose PQ when we shouldn't.

This appears to have been the decision we took on this: we won't change
the default; users affected by this can already turn
max_parallel_workers_per_gather to 0; most users benefit from it being
nonzero.

What we could probably do, is improve the observability around parallel
worker costs, so that people *know* that they need to do change the value.
What do users need to pay attention to? Are there metrics we should
expose?

At this point I think we should mark this commitfest entry as rejected.
https://commitfest.postgresql.org/patch/5751/

--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/