Re: Add 64-bit XIDs into PostgreSQL 15

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:

[skipped]

> > 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.

Probably, some table storage housekeeping would be wanted.

Like a column in pg_class describing the current set of options
of the table: checksums added, 64-bit xids added, type of 64-bit
xids (probably some would want to add support for the pgpro up-
grades), some set of defaults to not include a lot of them in all
pageheaders -- like compressed xid/integer formats or extended
pagesize.

And separate tables that describe the transition state --
like when adding checksums, the desired state for the relation
(checksums), and a set of ranges in the table files that are al-
ready transitioned/checked.

That probably will not introduce too much slowdown at least on
reading, and will add the transition/upgrade mechanics.

Aren't there were already some discussions about such a feature
in the mailing lists?

>
>
> --
> 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.
>
>