Re: Add 64-bit XIDs into PostgreSQL 15

On Fri, 7 Jan 2022 at 16:09, Justin Pryzby <pryzby@telsasoft.com> wrote:
>
> On Fri, Jan 07, 2022 at 03:53:51PM +0000, Finnerty, Jim wrote:
> > I'd still like a plan to retire the "double xmax" representation eventually.  Previously I suggested that this
couldbe done as a post-process, before upgrade is complete, but that could potentially make upgrade very slow. 
> >
> > Another way to retire the "double xmax" representation eventually could be to disallow "double xmax" pages in
subsequentmajor version upgrades (e.g. to PG16, if "double xmax" pages are introduced in PG15).  This gives the luxury
oftime after a fast upgrade to convert all pages to contain the epochs, while still providing a path to more
maintainablecode in the future. 
>
> Yes, but how are you planning to rewrite it?  Is vacuum enough?

Probably not, but VACUUM is the place to add such code.

> I suppose it'd need FREEZE + DISABLE_PAGE_SKIPPING ?

Yes

> This would preclude upgrading "across" v15.  Maybe that'd be okay, but it'd be
> a new and atypical restriction.

I don't see that restriction. Anyone upgrading from before PG15 would
apply the transform. Just because we introduce a transform in PG15
doesn't mean we can't apply that transform in later releases as well,
to allow say PG14 -> PG16.

--
Simon Riggs                http://www.EnterpriseDB.com/