Re: How to check whether a data type can be cast to another

Thank you Merlin and Tom. I appreciate your answers.

I have tried to follow that approach before I go this way. So, I checked
the appendix of the documentation for error conditions. Searching for
cast-related conditions, I found INVALID_CHARACTER_VALUE_FOR_CAST. Would
that be the exception to catch? It doesn't sound right.

The problem I faced is that the type 'any' is not permitted in
procedural languages. If it were permitted, then we could have a
function like is_castable( value ANY, target_type TEXT) and then we
would catch exceptions if they are thrown. We didn't want to begin
working on C functions before the prototype is complete. However, if
this is the only way, C is the way to go for now then.

If we can, programmatically, figure out whether a type can be cast to
another, we can then check each type's limits or valid values to
consider the valid.

Here is what we have, there is a field in which we should store
different data types and another field in the same row to store the
identifier of the original data type of the first field. The first field
should be polymorphic. Since I couldn't find how to make a field
polymorphic, I resorted to a 'text' field so that different data types
can be stored in the field after being cast to 'text'. When retrieving a
field, its original state should be cast back from 'text'.

Do you think of a better strategy?

Thank you, Merlin and Tom. I benefited from your answers. I appreciate
your help.

Regards,
Kareem

On 10/23/2009 04:00 PM, Tom Lane wrote:
> Kareem Sedki <isiscreation(at)gmail(dot)com> writes:
>
>> I am trying to write a function that takes one source and one target
>> data type identifiers, each as a '/text/' variable, as arguments and
>> return true if the cast can be made, false otherwise.
>>
> I think this is fundamentally the wrong approach, because it ignores the
> question of whether a specific value can be cast. For example, there is
> a cast from int4 to int2, but that doesn't mean that 999999::int2 will
> succeed. What I think you should do is just try the cast and catch any
> error that occurs.
>
> If you really want to try to do what you're saying, look into
> parser/parse_coerce.c. In particular, can_coerce_type() is the
> authoritative answer.
>
> regards, tom lane
>