Is it possible to upgrade from PostgreSQL 8.3.x to 8.4.x without downtime? We have a warm standby configuration with WAL shipping so we have the option upgrade the standby first, but WAL shipping does not apparently work from a 8.3.x primary to a 8.4.x slave or vice versa. Any suggestions would be appreciated as we have a mission critical app.
Other than that, I imagine you could put the database in read-only mode and perform the upgrade on the standby or another server.
I hope you well-tested this critical app on 8.4 already :)