Re: Not null contraints

On Sat, 14 Oct 2000, Philip Warner wrote:

> At 02:02 14/10/00 -0400, Tom Lane wrote:
> >>> Yup, that should do the trick.  Not much magic here...
> >
> >> Just to confirm - does this mean we have ALTER TABLE ADD CONSTRAINT, but
> >> not ALTER TABLE DROP CONSTRAINT. If so, should it go on a list somewhere?
> >
> >No, it just means that NOT NULL constraint is handled via a special
> >flag attached to the column's pg_attribute entry.
>
> So there is no 'ALTER TABLE ALTER <field> ALLOW NULLS' or whatever.

I wonder how you actually are supposed to add and remove NOT NULL
constraints.  ALTER TABLE ADD/DROP constraint work on table constraints,
and I don't think NOT NULL is among them, and I don't actually see
anything in spec beyond changing defaults and dropping for existing
columns.

>
> >Another relevant comment is that *removing* a NOT NULL constraint
> >doesn't pose any risk of creating invalid entries in the table data.
> >So there's no need to worry about cross-checking.
>
> This should apply to removing *any* constraint AFAICT...

True, but there might be cases in which removing a constraint invalidates
another one (removing the unique constraint that a foreign key constraint
references - not that we do anything about this yet)