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

On Sat, Jan 22, 2022 at 12:17 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Fri, Jan 21, 2022 at 8:40 AM Shruthi Gowda <gowdashru(at)gmail(dot)com> wrote:
> > From what I see in the code, template0 and postgres are the last
> > things that get created in initdb phase. The system OIDs that get
> > assigned to these DBs vary from release to release. At present, the
> > system assigned OIDs of template0 and postgres are 13679 and 13680
> > respectively. I feel it would be safe to assign 16000 and 16001 for
> > template0 and postgres respectively from the unpinned object OID range
> > 12000 - 16383. In the future, even if the initdb unpinned objects
> > reach the range of 16000 issues can only arise if initdb() creates
> > another system-created database for which the system assigns these
> > reserved OIDs (16000, 16001).
>
> It doesn't seem safe to me to rely on that. We don't know what could
> happen in the future if the number of built-in objects increases.
> Looking at the lengthy comment on this topic in transam.h, I see that
> there are three ranges:
>
> 1-9999 manually assigned OIDs
> 10000-11999 OIDs assigned by genbki.pl
> 12000-16384 OIDs assigned to unpinned objects post-bootstrap
>
> It seems to me that what this comment is saying is that OIDs in the
> second and third categories are doled out by counters. Therefore, we
> can't know which of those OIDs will get used, or how many of them will
> get used, or which objects will get which OIDs. Therefore, I think we
> should go back to the approach that you were using for template0 and
> handle both that database and postgres using that method. That is,
> assign an OID manually, and make sure unused_oids knows that it should
> be counted as already used.

Agree. In the latest patch, the template0 and postgres OIDs are fixed
to unused manually assigned OIDs 4 and 5 respectively. These OIDs are
no more listed as unused OIDs.

Regards,
Shruthi KC
EnterpriseDB: http://www.enterprisedb.com