Re: BUG #18343: Incorrect description in postgresql.conf for max_parallel_workers_per_gather

On Thu, Feb 15, 2024 at 8:55 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

PG Bug reporting form <noreply@postgresql.org> writes:
> In the default postgresql.conf that is generated, there are the following
> lines:

> #max_worker_processes = 8             # (change requires restart)
> #max_parallel_workers_per_gather = 2  # taken from max_parallel_workers
> <<<<<<<<<<<<<<<<<<<<<<<< THIS
> #max_parallel_maintenance_workers = 2 # taken from max_parallel_workers
> max_parallel_workers = 8              # maximum number of max_worker_processes that
>                                       # can be used in parallel operations

> it indicates that the default value is taken from whatever
> max_parallel_workers is.

No, you're misreading it.  There's no magic connection between these
two settings.  What the comment means to say is that the per-gather
worker processes come out of a pool of at most max_parallel_workers
processes.  Perhaps another wording would be better, but we don't have
a lot of space here --- any thoughts?

max_parallel workers = 8   # allocated from max_worker_processes

max_parallel_*_workers = N  # allocated from max_parallel_workers

or maybe "consumed from ..."

Or

max_parallel_*_workers = N # capped at max_parallel_workers

The last one turns a process-oriented description into a constraint, the latter seems to fit better in a config file.

David J.