Обсуждение: Casting from a domain

I'm trying to create a seconds domain that is an interval 'mashed to
seconds' as I once saw Tom describe it.

decibel=# create domain rrs.seconds as double precision;
CREATE DOMAIN
decibel=# create cast (double precision as rrs.seconds) WITHOUT FUNCTION AS IMPLICIT;
CREATE CAST
decibel=# create function rrs.interval_to_seconds(interval) returns seconds as 'SELECT extract( EPOCH FROM $1 );'
LANGUAGESQL; 
CREATE FUNCTION
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

\dC shows that the cast is there, and rrs.interval_to_seconds works as
expected, and according to \df does return seconds.

Version is 7.4.5.
--
Jim C. Nasby, Database Consultant               decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828

Windows: "Where do you want to go today?"
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Jim C. Nasby wrote:
> decibel=# create cast (interval as rrs.seconds) WITH FUNCTION rrs.interval_to_seconds(interval)  AS IMPLICIT;
                                      ^^^
> decibel=# select cast('1 month'::interval AS seconds);
> ERROR:  cannot cast type interval to seconds

It's not a schema issue is it? I.E. the lack of "rrs."?

--
   Richard Huxton
   Archonet Ltd

"Jim C. Nasby" <decibel@decibel.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

On Fri, Mar 04, 2005 at 03:40:17PM -0500, Tom Lane wrote:
> "Jim C. Nasby" <decibel@decibel.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.

Yeah, I figured it was probably an issue with it being a domain. I was
looking for a way to do this without the extra work involved in defining
a full-blown type. I guess the good news is I should be able to re-use
all the double-precision functions and what-not for 'seconds'.

BTW, is there a reason 'double' isn't an alias for 'double precision'?
I'm pretty sure every other database I've used (other than oracle of
course) supports 'double'. " precision" is just too much extra typing
after all... :P
--
Jim C. Nasby, Database Consultant               decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828

Windows: "Where do you want to go today?"
Linux: "Where do you want to go tomorrow?"
FreeBSD: "Are you guys coming, or what?"