pg_dump: ERROR: could not open relation with OID ...

During a routine backup procedure (that does not run nightly) for an
8.2.3 postgres cluster, pg_dump failed:

pg_dump: Error message from server: ERROR: could not open relation
with OID ...

In doing some log forensics, I discovered that this error has been
showing up in the logs intermittently unconnected to pg_dump for the
past 6 days. It's not occurring at an alarming rate, but the fact
that it's occurring at all is mildly alarming, and the fact that it's
preventing backups is even more alarming.

In reviewing the logs, one OID in particular shows up in the vast
majority of the errors, and it doesn't correspond to any entries I
can find in pg_class. A handful of other OIDs show up, and a sampling
of them reveals, too, no entries in pg_class.

The other curious item is that a number of the errors occur in a
pattern where they precede other error statements and don't seem to
be directly tied to connections, except during the failed pg_dumps.

The typical pattern in the logs is:

[timestamp] [pid] [remote host/port]:ERROR: [error message]
[timestamp] [pid] [remote host/port]:STATEMENT: [statement]

With this error, though, the format doesn't include the remote host/
port, which makes me wonder if it's occurring as a result of
autovacuum or other local/internal activity.

Now my thoughts return nervously to a previous thread:

http://archives.postgresql.org/pgsql-general/2007-05/msg01208.php

I didn't think much of it at the time, but now I wonder if it was
indicative of trouble on the way?

--
Thomas F. O'Connell

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