Re: Add 64-bit XIDs into PostgreSQL 15

On Tue, Jan 4, 2022 at 10:22:50PM +0000, Finnerty, Jim wrote:
> I'm concerned about the maintainability impact of having 2 new
> on-disk page formats. It's already complex enough with XIDs and
> multixact-XIDs.
>
> If the lack of space for the two epochs in the special data area is
> a problem only in an upgrade scenario, why not resolve the problem
> before completing the upgrade process like a kind of post-process
> pg_repack operation that converts all "double xmax" pages to
> the "double-epoch" page format? i.e. maybe the "double xmax"
> representation is needed as an intermediate representation during
> upgrade, but after upgrade completes successfully there are no pages
> with the "double-xmax" representation. This would eliminate a whole
> class of coding errors and would make the code dealing with 64-bit
> XIDs simpler and more maintainable.

Well, yes, we could do this, and it would avoid the complexity of having
to support two XID representations, but we would need to accept that
fast pg_upgrade would be impossible in such cases, since every page
would need to be checked and potentially updated.

You might try to do this while the server is first started and running
queries, but I think we found out from the online checkpoint patch that
having the server in an intermediate state while running queries is very
complex --- it might be simpler to just accept two XID formats all the
time than enabling the server to run with two formats for a short
period. My big point is that this needs more thought.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

If only the physical world exists, free will is an illusion.