do only critical work during single-user vacuum?

When a user must shut down and restart in single-user mode to run
vacuum on an entire database, that does a lot of work that's
unnecessary for getting the system online again, even without
index_cleanup. We had a recent case where a single-user vacuum took
around 3 days to complete.

Now that we have a concept of a fail-safe vacuum, maybe it would be
beneficial to skip a vacuum in single-user mode if the fail-safe
criteria were not met at the beginning of vacuuming a relation. This
is not without risk, of course, but it should be much faster than
today and once up and running the admin would have a chance to get a
handle on things. Thoughts?

-- 
John Naylor
EDB: http://www.enterprisedb.com