Andrew Dunstan <andrew@dunslane.net> writes:
> I wonder if we could have a boolean flag in pg_enum, indicating that
> setting an enum to that value was forbidden.
Yeah, but that still offers no coherent solution to the problem of
what happens if there's a table that already contains such a value.
It doesn't seem terribly useful to forbid new entries if you can't
get rid of old ones.
Admittedly, a DISABLE flag would at least offer a chance at a
race-condition-free scan to verify that no such values remain
in tables. But as somebody already mentioned upthread, that
wouldn't guarantee that the value doesn't appear in non-leaf
index pages. So basically you could never get rid of the pg_enum
row, short of a full dump and restore.
We went through all these points years ago when the enum feature
was first developed, as I recall. Nobody thought that the ability
to remove an enum value was worth the amount of complexity it'd
entail.
regards, tom lane