Andres Freund <andres@2ndquadrant.com> writes:
> I absolutely hate to suggest it, but what about storing the last
> vacuum's xmin horizon in the main table's pg_class.options in the back
> branches?
Not workable. This would require a non-in-place update of the table's
pg_class row (at least in cases where the option wasn't stored already).
Which would require VACUUM to have an XID, which would make a whole lot
of assumptions fall over.
regards, tom lane