Re: Add 64-bit XIDs into PostgreSQL 15

On Wed, Jan 5, 2022 at 06:12:26PM -0600, Justin Pryzby wrote:
> On Wed, Jan 05, 2022 at 06:51:37PM -0500, Bruce Momjian wrote:
> > 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
>
> I think you meant the online checksum patch. Which this reminded me of, too.
>
> https://commitfest.postgresql.org/31/2611/

Sorry, yes, I have checkpoint on my mind. ;-)

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

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