Re: Re: Changing the default value of an inherited column

On Thu, Mar 29, 2001 at 02:29:38PM +0100, Oliver Elphick wrote:
> Peter Eisentraut wrote:
> >Tom Lane writes:
> >
> >> It seems that in pre-7.0 Postgres, this works:
> >>
> >> create table one(id int default 1, descr text);
> >> create table two(id int default 2, tag text) inherits (one);
> >>
> >> with the net effect that table "two" has just one "id" column with
> >> default value 2.
> >
> >Although the liberty to do anything you want seems appealing at first, I
> >would think that allowing this is not correct from an OO point of view.
>
> I don't agree; this is equivalent to redefinition of a feature (=method) in
> a descendant class, which is perfectly acceptable so long as the feature's
> signature (equivalent to column type) remains unchanged.

The O-O principle involved here is Liskov Substitution: if the derived
table is used in the context of code that thinks it's looking at the
base table, does anything break?

Changing the default value of a column should not break anything,
because the different default value could as well have been entered
in the column manually.

Nathan Myers
ncm(at)zembu(dot)com