On 24 November 2014 at 21:04, Robert Haas <robertmh...@gmail.com> wrote:

> On Mon, Nov 24, 2014 at 3:26 PM, Thom Brown <t...@linux.com> wrote:
> > I haven't seen this mentioned anywhere (although it may have as I haven't
> > read through the entire history of it), but would others find it useful
> to
> > have ALTER SYSTEM support comments?
> Oh, please no.
> The main thing that caused us to have no way of modifying
> postgresql.conf via SQL for so many years is that it's not clear how
> you can sensibly rewrite a file with comments in it.  For example, the
> default postgresql.conf file has stuff like this in it:
> #variable = someval
> If variable gets set to a non-default value, you might want to
> uncomment that line, but now you have to parse the comments, which
> will be tedious and error-prone and sometimes make stupid decisions:
> #Back in days of yore when dinosaurs ruled the earth, we had
> #autovacuum_naptime=1h, but that turned out to be a bad idea.
> #
> #autovacuum_naptime=1min

I'm not sure this is an argument against supporting comments to
postgresql.auto.conf since it's specifically intended not to be edited by
humans, and commenting out a parameter will remove it from the file upon
further ALTER SYSTEM invocations anyway.

It would perhaps be OK to have comments in postgresql.conf.auto if
> they were designated in some way that told us which specific comment
> was associated with which specific setting.  But we need to be very
> careful not to design something that requires us to write a parser
> that can ferret out human intent from context clues.

Perhaps the parser could automatically remove any comment blocks which are
followed by a blank/empty line.

So if we had:

work_mem = '4MB'
# Set this lower as we're using a really fast disk interace
#seq_page_cost = '0.5'

# Set random_page_cost lower as we're using an SSD
random_page_cost = '1.0'
# This line has been manually added by a human without a newline
maintenance_work_mem = '1GB'

# This is an orphaned comment

I would expect the next modification to the file to cause reduce it to:

work_mem = '4MB'

# Set random_page_cost lower as we're using an SSD
random_page_cost = 1.0

# This line has been manually added
maintenance_work_mem = '1GB'


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