Gregory Stark <stark@enterprisedb.com> writes:
> So if it's possible for the frozenxid in the visibility map to go backwards
> then it's no good, since if that update is lost we might skip a necessary
> vacuum freeze.
Seems like a lost disk write would be enough to make that happen.
Now you might argue that the odds of that are no worse than the odds of
losing an update to one particular heap page, but in this case the
single hiccup could lead to losing half a gigabyte of data (assuming 8K
page size). The leverage you get for saving vacuum freeze work is
exactly equal to the magnification factor for data loss.
regards, tom lane