Weird error

I have a Postgres application running right now.  The thing is
constantly doing 3-5 updates/sec and 1-2 multi-join selects/sec and
performance is actually doing all right.  Unfortunately, as the system
runs, performance degrades, which I guess has been documented, although
I still don't understand why.

To work around this, I have a cron job that runs every hour and vacuum
analyzes the three tables that are actually updated significantly.  Most of the time, it works fine, but recently, I've
beengetting this error: 

  NOTICE:  Child itemid in update-chain marked as unused - can't continue
  repair_frag

What causes this and how do I make it stop?  When this happens,
whatever table is affected doesn't get analyzed and the database
continues its downward resource spiral.

Thanks in advance,
Philip

* Philip Molter
* DataFoundry.net
* http://www.datafoundry.net/
* philip@datafoundry.net