- Original Message -
From: Darren Duncan dar...@darrenduncan.net
To: Jaime Casanova ja...@2ndquadrant.com
Cc: PostgreSQL-development pgsql-hackers@postgresql.org
Sent: Saturday, May 14, 2011 9:42 PM
Subject: Re: [HACKERS] DOMAINs and CASTs
Jaime Casanova wrote:
If i create a DOMAIN an then want to create a CAST from that domain to
another type it gives an error.
Consider this example:
create domain datetime as timestamp with time zone
check (value between '1753-01-01 00:00:00' and '-12-31 23:59:59');
create function datetime2int(datetime) returns int
language sql stable strict as $$
select $1::date - '1753-01-01'::date;
$$;
create cast(datetime as int) with function datetime2int(datetime);
if i try to cast, get this error:
select now()::datetime::int;
ERROR: cannot cast type datetime to integer
The problem is that in find_coercion_pathway() the very first thing we
do is to get the base type of both: the source and target types. So,
the way to make it work is to create the function and the cast on the
base types.
But what if i create 2 domains on the same base types and want a
different behaviour on a cast to the same target type?
I think that overloading the same cast syntax to get different behavior
for different domains over the same base type is a bad idea.
First of all, what if cast(timestamp as int) was already defined? Which
cast then would you expect to be invoked here?
'1800-01-01 00:00:00'::int
... the one for timestamp or the one for datetime?
Second of all, what if you had 2 domains defined over timestamp and they
overlapped and they both defined a cast as you did, with generic syntax?
And you were casting a value in both domains as an int?
I think it would be best that the generic cast syntax only be useable for
casts defined on the base type, and if you want a domain-specific one you
should use the function syntax such as your datetime2int().
That way it is easier for users to predict what behavior will occur, and
implementation will be easier too.
-- Darren Duncan
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