Re: Add 64-bit XIDs into PostgreSQL 15

>
> Did you consider using 4 bytes for pd_xid_base and another 4 bytes for
> (pd_xid_base/pd_multi_base)? This would allow calculating XMIN/XMAX
> as:
>
> XMIN = (t_min_extra_bits << 32) | t_xmin
> XMAX = (t_max_extra_bits << 32) | t_xmax
>
> ... and save 8 extra bytes in the pd_special area. Or maybe I'm
> missing some context here?
>
Hi, Alexander!

In current design it is not possible, as pd_xid_base is roughly just a
minimum 64-xid of all tuples that may fit this page. So we do not make any
extra guess that it should be in multiples of 2^32.

If we make pd_xid_base in multiples of 2^32 then after current XID crosses
the border of 2^32 then pages that contains tuples with XMIN/XMAX before
this point are not suitable for tuple inserts anymore. In effect we will
then have "sets" of the pages for each 2^32 "epoch" with freed space that
can not be used anymore.

I think it's too big a loss for gain of just 8 bytes per page.

Thank you for your dive into this matter!

--
Best regards,
Pavel Borisov

Postgres Professional: http://postgrespro.com <http://www.postgrespro.com>