The database objects (tables, indexes, etc.) that existed
as of time t0 may have changed so that db.db does
not equal db.db0. db.db continues
to read data from dbspaces A, B and C as
long as the tables that existed at time t0 exist
and as long as they contain some unmodified rows of data that existed
as of time t0. Even if or when this is no longer
true, db.db will continue to open A, B and C unless
they are dropped from db.db, which is only allowed if they are empty
from db.db's point of view.