True. So if your goal is to force the timestamp column to be the correct value even when the user tries to set it to something else, you'd still have to use a trigger or rule.
Maybe the rule is that the computed value is always used, unless: UPDATE foo OVERRIDE DEFAULTS set d=yesterday();
*shrug*. At least with something like the above, the user makes his intention explicit. Perhaps if user doesn't specify OVERRIDE DEFAULTS, postgres outputs a warning:
WARNING: value for column 'd' ignored.
HINT: Use UPDATE ... OVERRIDE DEFAULTS to override ON UPDATE DEFAULT values
and of course, this would be handy too:
UPDATE foo OVERRIDE DEFAULTS set d=DEFAULT;eric
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