Vlad Romascanu <vromascanu@accurev.com> writes:
> Hi, Tom,
> Why does:
> CREATE CAST (text AS bytea) WITHOUT FUNCTION;
> SELECT E'C:\\something'::text::bytea;
> work as expected, but (with the original text->bytea cast in place):
> CREATE DOMAIN my_varlena AS text;
> CREATE CAST (my_varlena AS bytea) WITHOUT FUNCTION;
> SELECT E'C:\\something'::my_varlena::bytea;
> does not
Domains are not meant to support ad-hoc cast paths like that ---
generally, the parser smashes domains to their base types before even
looking into pg_cast. The reason for this is that the defined pathway
is source_domain -> source_base_type -> dest_base_type -> dest_domain
and allowing user-defined cast paths to short-circuit that would create
all kinds of uncertainty, in particular whether or not constraints on
a destination domain had been verified.
Possibly we ought to disallow CREATE CAST involving a domain, since
you're not the first person to think that he can impose special cast
rules by using a domain.
regards, tom lane