Re: Improving the "Routine Vacuuming" docs

On Tue, Apr 12, 2022 at 5:53 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> My high level concerns are:
>
> * Instead of discussing FrozenTransactionId (and then explaining how
> that particular magic value is not really used anymore anyway), why
> not describe freezing in terms of the high level rules?
>
> Something along the lines of the following seems more useful: "A tuple
> whose xmin is frozen (and xmax is unset) is considered visible to
> every possible MVCC snapshot. In other words, the transaction that
> inserted the tuple is treated as if it ran and committed at some point
> that is now *infinitely* far in the past."

I agree with this idea.

> * The description of wraparound sounds terrifying, implying that data
> corruption can result.
>
> The alarming language isn't proportionate to the true danger
> (something I complained about in a dedicated thread last year [1]).

I mostly agree with this, but not entirely. The section needs some
rephrasing, but xidStopLimit doesn't apply in single-user mode, and
relfrozenxid and datfrozenxid values can and do get corrupted. So it's
not a purely academic concern.

> * XID space isn't really a precious resource -- it isn't even a
> resource at all IMV.

I disagree with this. Usable XID space is definitely a resource, and
if you're in the situation where you care deeply about this section of
the documentation, it's probably one in short supply. Being careful
not to expend too many XIDs while fixing the problems that have cause
you to be short of safe XIDs is *definitely* a real thing.

> * We don't cleanly separate discussion of anti-wraparound autovacuums,
> and aggressive vacuums, and the general danger of wraparound (by which
> I actually mean the danger of having the xidStopLimit stop limit kick
> in).

I think it is wrong to conflate wraparound with xidStopLimit.
xidStopLimit is the final defense against an actual wraparound, and
like I say, an actual wraparound is quite possible if you put the
system in single user mode and then do something like this:

backend> VACUUM FULL;

Big ouch.

--
Robert Haas
EDB: http://www.enterprisedb.com