Re: The suggestion of reducing autovacuum_vacuum_cost_delay should be documented

On Tue, May 12, 2020 at 07:26:07PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > On Tue, May 12, 2020 at 03:38:16PM -0700, David G. Johnston wrote:
> >> The proposal is to document in versions 9.4 to 11 that the recommended value
> >> for the setting is 2ms while for reasons of continuity the default in these
> >> versions is 20ms.
> >> I don't really see any harm in it.  Its not like the choice to reduce the value
> >> was made because of new features introduced in 12 - it was a re-evaluation of a
> >> 15 year old default.
>
> > Well, we really need to have some general discussion about whether
> > changing defaults in major releases should trigger a mention to change
> > the defaults in back branches.  This is something that would have to be
> > discussed on the hackers list.
>
> It's not immediately obvious that the new default value established in
> version N is appropriate for version N-minus-several.  Certainly, whatever
> testing was done to justify the new default wouldn't have been done on old
> versions; and there might have been relevant changes.
>
> In short: nope, I'm not on board with blindly back-patching such
> recommendations.

That was my analysis too.