Paul,

P> The real issue is that a field has changed how it works, so you really
P> need to review all code that uses that field, to see if the software is
P> compatible with the new use. For example, suppose there is an old
P> program, it works, but nobody can change it, because it uses some weird
P> library, that you can't find, and the publisher went out of business.
P> This program intentionally make use of the NULL on that field, so
P> removing the NULL will break this program.

(fortunately) nobody has considered this POV really important when they
implemented ALTER TABLE and ALTER COLUMN. I don't think it's
reasonable to require a DBMS to NOT offer the ability to change
metadata just because some old app could need it as it was. That's a
concern for the user, not the DBMS.

All your other arguments I agree with, and they were discussed already
on the lists, I believe.

Ciao
--
Nando Dessena
mailto:nandod@...