Re: Casting from a domain

"Jim C. Nasby" <decibel(at)decibel(dot)org> writes:
> decibel=# create cast (interval as rrs.seconds) WITH FUNCTION rrs.interval_to_seconds(interval) AS IMPLICIT;
> CREATE CAST
> decibel=# select cast('1 month'::interval AS seconds);
> ERROR: cannot cast type interval to seconds

Given the current coercion rules, we should probably disallow attempts
to define casts that involve domains. Casts are on base types. The
down-cast from a domain to its base type is hardwired, and the up-cast
from base type to domain is too (with invocation of any constraints
that may apply). Adding random user-defined casts to this would
probably just create confusion and ambiguity. In particular, this
was already meaningless:

decibel=# create domain rrs.seconds as double precision;
CREATE DOMAIN
decibel=# create cast (double precision as rrs.seconds) WITHOUT FUNCTION AS IMPLICIT;
CREATE CAST

since the presence of the cast might be thought to justify coercing
floats to "seconds" without invoking domain constraints.

regards, tom lane