Re: Add 64-bit XIDs into PostgreSQL 15

Hi Pavel,

> Please feel free to discuss readme and your opinions on the current patch and proposed changes [1].

Just a quick question about this design choice:

> On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
> lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
> 64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
> They are placed into a pd_special area - 16 bytes in the end of a heap page.
> Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
> as follows:
>
> XMIN = t_xmin + pd_xid_base.
> XMAX = t_xmax + pd_xid_base/pd_multi_base.

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?

--
Best regards,
Aleksander Alekseev