Re: The "prepare" approach was the first tried.
https://github.com/postgrespro/pg_pageprep
But it appears to be very difficult and unreliable. After investing
many months into pg_pageprep, "double xmax" approach appears to be
very fast to implement and reliable.
I'd still like a plan to retire the "double xmax" representation eventually.
Previously I suggested that this could be done as a post-process, before
upgrade is complete, but that could potentially make upgrade very slow.
Another way to retire the "double xmax" representation eventually could be to
disallow "double xmax" pages in subsequent major version upgrades (e.g. to
PG16, if "double xmax" pages are introduced in PG15). This gives the luxury of
time after a fast upgrade to convert all pages to contain the epochs, while
still providing a path to more maintainable code in the future.
