Re: make MaxBackends available in _PG_init

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> Gah, I hate putting this off for another year, but I guess I'm also
> not convinced that 0003 has the right idea, so maybe it's for the
> best. Here's what's bugging me: 0003 decrees that _PG_init() is the
> Wrong Place to Adjust GUC Settings. Now, that begs the question: what
> is the right place to adjust GUC settings? I argued upthread that
> extensions shouldn't be overriding values from postgresql.conf at all,
> but on further reflection I think there's at least one legitimate use
> case for this sort of thing: some kind of auto-tuning module that, for
> example, looks at how much memory you have and sets shared_buffers or
> work_mem or something based on the result.

Yeah. It's a very long way from "changing shared memory sizes here
doesn't work" to "no extension is allowed to change any GUC at load
time". A counterexample is that it's not clear why an extension
shouldn't be allowed to define a GUC using DefineCustomXXXVariable
and then immediately set it to some other value. I think trying to
forbid that across-the-board is going to mostly result in breaking
a lot of cases that are perfectly safe.

regards, tom lane