Re: making relfilenodes 56 bits

On Tue, Aug 23, 2022 at 8:00 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Tue, Aug 23, 2022 at 2:06 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > OTOH, if we keep the two separate ranges for the user and system table
> > then we don't need all this complex logic of conflict checking.
>
> True. That's the downside. The question is whether it's worth adding
> some complexity to avoid needing separate ranges.
>
> Honestly, if we don't care about having separate ranges, we can do
> something even simpler and just make the starting relfilenumber for
> system tables same as the OID. Then we don't have to do anything at
> all, outside of not changing the OID assigned to pg_largeobject in a
> future release. Then as long as pg_upgrade is targeting a new cluster
> with completely fresh databases that have not had any system table
> rewrites so far, there can't be any conflict.
>
> And perhaps that is the best solution after all, but while it is
> simple in terms of code, I feel it's a bit complicated for human
> beings. It's very simple to understand the scheme that Amit proposed:
> if there's anything in the new cluster that would conflict, we move it
> out of the way. We don't have to assume the new cluster hasn't had any
> table rewrites. We don't have to nail down starting relfilenumber
> assignments for system tables. We don't have to worry about
> relfilenumber or OID assignments changing between releases.
> pg_largeobject is not a special case. There are no special ranges of
> OIDs or relfilenumbers required. It just straight up works -- all the
> time, no matter what, end of story.
>

This sounds simple to understand. It seems we always create new system
tables in the new cluster before the upgrade, so I think it is safe to
assume there won't be any table rewrite in it. OTOH, if the
relfilenumber allocation scheme is robust to deal with table rewrites
then we probably don't need to worry about this assumption changing in
the future.

--
With Regards,
Amit Kapila.