Re: Challenges preventing us moving to 64 bit transaction id (XID)?

On Tue, Jun 6, 2017 at 09:05:03AM -0400, Peter Eisentraut wrote:
> On 6/6/17 08:29, Bruce Momjian wrote:
> > On Tue, Jun 6, 2017 at 06:00:54PM +0800, Craig Ringer wrote:
> >> Tom's point is, I think, that we'll want to stay pg_upgrade
> >> compatible. So when we see a pg10 tuple and want to add a new page
> >> with a new page header that has an epoch, but the whole page is full
> >> so there isn't 32 bits left to move tuples "down" the page, what do we
> >> do?
> >
> > I guess I am missing something. If you see an old page version number,
> > you know none of the tuples are from running transactions so you can
> > just freeze them all, after consulting the pg_clog. What am I missing?
> > If the page is full, why are you trying to add to the page?
>
> The problem is if you want to delete from such a page. Then you need to
> update the tuple's xmax and stick the new xid epoch somewhere.
>
> We had an unconference session at PGCon about this. These issues were
> all discussed and some ideas were thrown around. We can expect a patch
> to appear soon, I think.

Sorry I missed the unconference session.

OK, crazy idea. Since we know the creation is frozen can we put the
epoch in the xmin and set some tuple bit that only has meaning on old
page versions? Yeah, I said crazy.

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

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