Somebody (I think Joe or Heikki) poked a big hole in this last night at
the Royal Oak.  Although the scheme would get rid of the need to replace
old XIDs with FrozenXid, it does not get rid of the need to set hint
bits before you can truncate CLOG.  So in your example of an insert-only
table that's probably never read again, there's still a minimum of one
update visit required on every old page.  Now that's still better than
two update visits ... but we could manage that already, just by tweaking
vacuum's heuristics about when to freeze vs when to set hint bits.

Yeah, someone pointed that out to me too and suggested that a freeze map was the better solution. I still think there's something we can do with pages on the visibility map but I'll have to think about it some more.

                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.

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