Re: Add pg_basetype() function to obtain a DOMAIN base type

On Thu, Mar 21, 2024 at 10:34 AM jian he <jian.universality@gmail.com> wrote:
>
> On Mon, Mar 18, 2024 at 11:43 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >
> > Alexander Korotkov <aekorotkov@gmail.com> writes:
> > > On Mon, Mar 18, 2024 at 2:01 AM jian he <jian.universality@gmail.com> wrote:
> > >> `
> > >> Datum
> > >> pg_basetype(PG_FUNCTION_ARGS)
> > >> {
> > >>      Oid oid;
> > >>
> > >>      oid =  get_fn_expr_argtype(fcinfo->flinfo, 0);
> > >>      PG_RETURN_OID(getBaseType(oid));
> > >> }
> > >> `
> >
> > > Looks good to me.  But should it be named pg_basetypeof()?
> >
> > I still don't like this approach.  It forces the function to be
> > used in a particular way that's highly redundant with pg_typeof.
> > I think we'd be better off with
> >
> > pg_basetype(PG_FUNCTION_ARGS)
> > {
> >         Oid typid = PG_GETARG_OID(0);
> >
> >         PG_RETURN_OID(getBaseType(typid));
> > }
> >
> > The use-case that the other definition handles would be implemented
> > like
> >
> >         pg_basetype(pg_typeof(expression))
> >
>
> trying to do it this way.
> not sure the following error message is expected.
>
> SELECT pg_basetype(-1);
> ERROR:  cache lookup failed for type 4294967295

I think the error message should be fine.
even though
`select '-1'::oid::regtype;` return 4294967295.

I noticed psql \dD didn't return the basetype of a domain.
one of the usage of this feature would be in psql \dD.

now we can:
\dD mytext_child_2
                                       List of domains
 Schema |      Name      |      Type      | Basetype | Collation |
Nullable | Default | Check
--------+----------------+----------------+----------+-----------+----------+---------+-------
 public | mytext_child_2 | mytext_child_1 | text     |           |
     |         |
(1 row)