Re: preserving db/ts/relfilenode OIDs across pg_upgrade (was Re: storing an explicit nonce)

Greetings,

* Robert Haas (robertmhaas@gmail.com) wrote:
> On Thu, Aug 26, 2021 at 11:39 AM Stephen Frost <sfrost@snowman.net> wrote:
> > This looks like a pretty good analysis to me.  As it relates to the
> > question about allowing users to specify an OID, I'd be inclined to
> > allow it but only for OIDs >64k.  We've certainly reserved things in the
> > past and I don't see any issue with having that reservation here, but if
> > we're going to build the capability to specify the OID into CREATE
> > DATABASE then it seems a bit odd to disallow users from using it, as
> > long as we're preventing them from causing problems with it.
> >
> > Are there issues that you see with allowing users to specify the OID
> > even with the >64k restriction..?  I can't think of one offhand but
> > perhaps I'm missing something.
> 
> So I actually should have said 16k here, not 64k, as somebody already
> pointed out to me off-list. Whee!

Hah, yes, of course.

> I don't know of a reason not to let people do that, other than that it
> seems like an attractive nuisance. People will do it and it will fail
> because they chose a duplicate OID, or they'll complain that a regular
> dump and restore didn't preserve their database OIDs, or maybe they'll
> expect that they can copy a database from one cluster to another
> because they gave it the same OID! That said, I don't see a great harm
> in it. It just seems to me like exposing knobs to users that don't
> seem to have any legitimate use may be borrowing trouble.

We're going to have to gate this somehow to allow the OIDs under 16k to
be used, so it seems like what you're suggesting is that we have that
gate in place but then allow any OID to be used if you've crossed that
gate?

That is, if we do something like:

SELECT pg_catalog.binary_upgrade_allow_setting_db_oid();
CREATE DATABASE blah WITH OID 1234;

for pg_upgrade, well, users who are interested may well figure out how
to do that themselves if they decide they want to set the OID, whereas
if it 'just works' provided they don't try to use an OID too low then
maybe they won't try to bypass the restriction against using system
OIDs..?

Ok, I'll give you that this is a stretch and I'm on the fence about if
it's worthwhile or not to include and document and if, as you say, it's
inviting trouble to allow users to set it.  Users do seem to have a
knack for finding things even when they aren't documented and then we
get to deal with those complaints too. :)

Perhaps others have some stronger feelings one way or another.

Thanks,

Stephen