Jeff Davis wrote:
> On Fri, 2011-04-08 at 07:08 -0400, Noah Misch wrote:
> > > Right, VACUUM FREEZE. I now see I don't need to set
> > > vacuum_freeze_table_age if I use the FREEZE keyword, e.g. gram.y has:
> > >
> > > if (n->options & VACOPT_FREEZE)
> > > n->freeze_min_age = n->freeze_table_age = 0;
> >
> > True; it just performs more work than strictly necessary. We don't actually
> > need earlier-than-usual freezing. We need only ensure that the relfrozenxid
> > will guide future VACUUMs to do that freezing early enough. However, I'm not
> > sure how to do that without directly updating relfrozenxid, so it's probably
> > just as well to cause some extra work and stick to the standard interface.
>
> If there are tuples in a toast table containing xids that are older than
> the toast table's relfrozenxid, then there are only two options:
>
> 1. Make relfrozenxid go backward to the right value. There is currently
> no mechanism to do this without compiling C code into the server,
> because (a) VACUUM FREEZE will never move the relfrozenxid backward; and
> (b) there is no way to find the oldest xid in a table with a normal
> snapshot.
Right, this is all to complicated.
> 2. Get rid of those xids older than relfrozenxid (i.e. VACUUM FREEZE).
>
> I don't know what you mean about VACUUM FREEZE doing extra work. I
> suppose you could set the vacuum_freeze_min_age to be exactly the right
> value such that it freezes everything before the existing (and wrong)
> relfrozenxid, but in practice I think it would be the same amount of
> work.
We don't know how far back to go with freezing, so we just have to
freeze it all.
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ It's impossible for everything to be true. +