Specific questions about wraparound and vacuum

Hi-

I have a few specific questions about wraparound that I'm not finding
answers for in the Docs or list archives. We're a few versions behind
(sorry!) at 7.4.7. Our database is large (100 GB), with most of the rows
concentrated in 5 tables, the largest containing rows.

1) Do we need to do a *full* vacuum or just a "lazy" vacuum to take care of
the xid wraparound issues? The documentation at
http://www.postgresql.org/docs/7.4/interactive/maintenance.html#VACUUM-FOR-WRAPAROUNDimplies
that a regular vacuum is all that is needed but is not explicit
about it. We vacuum nightly, but due to availability requirements almost
never do a full vacuum.

2) If a regular (non-full) vacuum will not reset the XID. Will a
dump/restore take care of wraparound? We have done this in the past for
space reclamation because we seem to be able to dump/restore more quickly
than we can do a full vacuum.

3) How can we tell if our attempts to take care of wraparound have worked? I
found a note that this select should tell me if we're in trouble:

SELECT datname, age(datfrozenxid) FROM pg_database;

But after a vaccum of both our prod and the template1 database, I get this
result:

datname | age
-----------+------------
prod | 1074324475
template1 | 1073742599
template0 | 363178963
(3 rows)

From the docs, I learned that after a vacuum, I should see this number at
one billion and get alarmed as I near 2 billion. This results is after about
3 hours of normal activity, and I'm almost one-tenth of the way to 2
billion. This scares me a bit, as it implies that after about 36 hours, I
would hit the wall if I don't run vacuum again.

Or... is it the case that by one billion, the docs really mean 2^30
(1073741824) ?

If the select above is not right, what should I be using to track how close
we are to wraparound problems?

4) The documentation implies that I need to vacuum *every* database, even if
I'm not using them all, so I vacuumed template1, but can't connect to
template0. Do I really need to vacuum databases where no transactions are
occurring? If so, how to I deal with template0?

Thanks.
-Nick
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Nick Fankhauser
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http://www.doxpop.com
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