@marcin and @tomek discussed a concept of wiping database. This may address several issues:
system tests need to discard all data from a DB after a CB test.
after schema was extended, every unit-test creates and then tears down the schema. With many (around 40) tables, this is very slow process. We should have an option (possibly even enabled by default) to wipe data from the tables rather then tear them down and recreate every time. (TRUNCATE TABLE is supposedly faster than DELETE FROM, but has some constraints and can't be used everywhere).