... 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.