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  1. Doc: improve a couple of comments in postgresql.conf.sample.

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

    The Post Office <noreply@postgresql.org> — 2024-02-14T22:11:14Z

    The following bug has been logged on the website:

Bug reference:      18343
Logged by:          Christopher Kline
Email address:      kline.christopher@gmail.com
PostgreSQL version: 14.11
Operating system:   Windows 10 x64
Description:        

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. However, if I start postgresql with those settings
and issue a query of 
   SELECT setting, unit FROM pg_settings WHERE name =
'max_parallel_workers_per_gather'

the result I get is '2', not '8'. 

This leads me to believe that either there is a bug in the code that's not
setting the correct default, or the comment in postgresql.conf is incorrect.

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

    Tom Lane <tgl@sss.pgh.pa.us> — 2024-02-15T15:55:10Z

    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?

			regards, tom lane

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

    David G. Johnston <david.g.johnston@gmail.com> — 2024-02-15T16:10:00Z

    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.

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

    Christopher Kline <kline.christopher@gmail.com> — 2024-02-15T16:29:32Z

    Thank you all for the clarification. I like David Johnston's suggestion of

# capped at max_parallel_workers


That clearly defines the constraint.

On Thu, Feb 15, 2024 at 11:10 AM David G. Johnston <
david.g.johnston@gmail.com> wrote:

> 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.
>
>

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

    Tom Lane <tgl@sss.pgh.pa.us> — 2024-02-15T17:31:33Z

    Christopher Kline <kline.christopher@gmail.com> writes:
> Thank you all for the clarification. I like David Johnston's suggestion of
> # capped at max_parallel_workers
> That clearly defines the constraint.

I was thinking perhaps "# limited by max_parallel_workers"
or something like that.  "Capped at" isn't phraseology we
use elsewhere.

			regards, tom lane

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

    David G. Johnston <david.g.johnston@gmail.com> — 2024-02-15T19:41:59Z

    On Thu, Feb 15, 2024 at 10:31 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Christopher Kline <kline.christopher@gmail.com> writes:
> > Thank you all for the clarification. I like David Johnston's suggestion
> of
> > # capped at max_parallel_workers
> > That clearly defines the constraint.
>
> I was thinking perhaps "# limited by max_parallel_workers"
> or something like that.  "Capped at" isn't phraseology we
> use elsewhere.
>
>
"limited by" is indeed better IMO as well.

David J.

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

    Christopher Kline <kline.christopher@gmail.com> — 2024-02-15T20:29:06Z

    Agreed.

On Thu, Feb 15, 2024 at 2:42 PM David G. Johnston <
david.g.johnston@gmail.com> wrote:

> On Thu, Feb 15, 2024 at 10:31 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
>> Christopher Kline <kline.christopher@gmail.com> writes:
>> > Thank you all for the clarification. I like David Johnston's suggestion
>> of
>> > # capped at max_parallel_workers
>> > That clearly defines the constraint.
>>
>> I was thinking perhaps "# limited by max_parallel_workers"
>> or something like that.  "Capped at" isn't phraseology we
>> use elsewhere.
>>
>>
> "limited by" is indeed better IMO as well.
>
> David J.
>

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

    Tom Lane <tgl@sss.pgh.pa.us> — 2024-02-15T21:30:39Z

    Christopher Kline <kline.christopher@gmail.com> writes:
> On Thu, Feb 15, 2024 at 2:42 PM David G. Johnston <
> david.g.johnston@gmail.com> wrote:
>> "limited by" is indeed better IMO as well.

> Agreed.

Sold, I'll make it so.

			regards, tom lane