Cache lookup failure for index during pg_dump

I have a job that summarizes some data from a table, truncates the table, drops the indexes, writes the summarized data back into the table, then recreates the indexes. The operations above are not in a single transaction, but separate statements executed by a script. Easy, runs great, has for years.

Recently the job takes a little longer to run and is still going when the database dump starts. That's when I started getting this:

ERROR: cache lookup failed for index 70424
STATEMENT: SELECT t.tableoid, t.oid, t.relname AS indexname, pg_catalog.pg_get_indexdef(i.indexrelid) AS indexdef, t.relnatts AS indnkeys, i.indkey, i.indisclustered, c.contype, c.conname, c.tableoid AS contableoid, c.oid AS conoid, (SELECT spcname FROM pg_catalog.pg_tablespace s WHERE s.oid = t.reltablespace) AS tablespace, array_to_string(t.reloptions, ', ') AS options FROM pg_catalog.pg_index i JOIN pg_catalog.pg_class t ON (t.oid = i.indexrelid) LEFT JOIN pg_catalog.pg_depend d ON (d.classid = t.tableoid AND d.objid = t.oid AND d.deptype = 'i') LEFT JOIN pg_catalog.pg_constraint c ON (d.refclassid = c.tableoid AND d.refobjid = c.oid) WHERE i.indrelid = '56448'::pg_catalog.oid ORDER BY indexname

The oid value changes daily, of course. pg_dump then disconnects and stops. I checked the source code and the query is definitely coming from pg_dump. When I run the dump a few minutes later it works beautifully, so I don't think the system catalogs are corrupt.

My questions are: can making DDL changes during a dump cause this error? Are the queries used by pg_dump transactionally consistent, i.e. do they run in a transaction and get a single view of the database system catalogs? Other than finer coordination of jobs, how can this situation be avoided?

I'm running PG 8.4.1 on linux.

Thanks in advance for your responses.

Bob Lunney