Re: do only critical work during single-user vacuum?

Hi,

On 2021-12-20 17:17:26 -0800, Peter Geoghegan wrote:
> On Thu, Dec 9, 2021 at 8:41 PM Bossart, Nathan <bossartn(at)amazon(dot)com> wrote:
> > I like the idea of having a built-in function that does the bare
> > minimum to resolve wraparound emergencies, and I think providing some
> > sort of simple progress indicator (even if rudimentary) would be very
> > useful.
>
> If John doesn't have time to work on this during the Postgres 15
> cycle, and if nobody else picks it up, then we should at least do the
> bare minimum here: force the use of the failsafe in single user mode
> (regardless of the age of relfrozenxid/relminmxid, which in general
> might not be that old in tables where VACUUM might need to do a lot of
> work). Attached quick and dirty patch shows what this would take. If
> nothing else, it seems natural to define running any VACUUM in single
> user mode as an emergency.

As I said before I think this is a bad idea. I'm fine with adding a vacuum
parameter forcing failsafe mode. And perhaps a hint to suggest it in single
user mode. But forcing it is a bad idea - single user isn't just used for
emergencies (c.f. initdb, which this patch would regress) and not every
emergency making single user mode useful is related to wraparound.

Greetings,

Andres Freund