I have never worked with a database of that size but I have done major refactoring of tables with about half a billion rows. And while it can take hours for the schema changes to finish it is hours without any downtime (or just a couple of seconds of downtime during when the exclusive locks need to be acquired). It is tedious to have to wait that long (especially if one has 10 times the rows than what we did), but PostgreSQL can do almost any change online just fine.

What kind of schema changes do you think of that would require that much downtime? Even in complex cases that can be avoided with triggers or even rules.