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

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.

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