Re: Allow deleting enumerated values from an existing enumerated data type

On 9/28/23 20:46, Tom Lane wrote:
> 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.

This issue comes up regularly (although far from often).  Do we want to 
put some comments right where would-be implementors would be sure to see it?

Attached is an example of what I mean.  Documentation is intentionally 
omitted.
-- 
Vik Fearing