Re: We should stop telling users to "vacuum that database in single-user mode"

On Mon, Mar 01, 2021 at 04:32:23PM +0100, Hannu Krosing wrote:
> It looks like we are unnecessarily instructing our usiers to vacuum their
> databases in single-user mode when just vacuuming would be enough.

> When I started looking at improving the situation I discovered, that there
> already is no need to run VACUUM in single user mode in any currently supported
> PostgreSQL version as you can run VACUUM perfectly well when the wraparound
> protection is active.

A comment in SetTransactionIdLimit() says, "VACUUM requires an XID if it
truncates at wal_level!=minimal." Hence, I think plain VACUUM will fail some
of the time; VACUUM (TRUNCATE false) should be reliable. In general, I like
your goal of replacing painful error message advice with less-painful advice.
At the same time, it's valuable for the advice to reliably get the user out of
the bad situation.