Re: autovacuum_freeze_max_age

Jacques Caron <jc(at)oxado(dot)com> writes:
> I was wondering why autovacuum_freeze_max_age actually can't be set
> at runtime... guc.c points to varsup.c which says:

Hmm. The original worry was that different uses might be out of sync,
but right offhand it doesn't look like anything catastrophic would
result from that. At least not with the current code; it's unclear
whether it'd stay safe in the face of changes.

> I'm not sure I understand what would need to be updated other than
> the variable itself? Obviously it would mean that it would only take
> effect the next time SetTransactionIdLimit is called, i.e. the next
> time VACUUM is run, but I guess this is always better than having to
> restart the database?

Uh, no, it's called only when we've advanced datfrozenxid,
which is a pretty uncommon event. So I think the real hole in the
proposal is that there would be a long and not-very-predictable delay
between changing the parameter and having it really take effect.

If there were a big use-case for changing this setting on the fly then
I'd be more worried about it, but I don't see one really ...

regards, tom lane