Kareem Sedki <isiscreation@gmail.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