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 been getting 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(at)datafoundry(dot)net