(I have
heard, more often than I care to think, that the schema in an RDBMS is
effectively impossible to change, although one of the key innovations in
the development of RDBMS was precisely that it's easier to change the
schema than previously).
Easier to change because data independence meant changes in the physical structures of the database weren't exposed to the applications.
People who never experienced schema changes in pre-relational databases (which has now become the norm) don't appreciate it.
