Luca Ferrari <fluca1978(at)infinito(dot)it> writes:
> ok this is a silly question, but I've got a doubt: what is the exact meaning
> of Datum?
It's the backend-internal representation of a single value of any SQL
data type. The code using the Datum has to know which type it is,
since the Datum itself doesn't contain that information. Usually,
C code will work with a value in a "native" representation, and then
convert to or from Datum in order to pass the value through
data-type-independent interfaces.
regards, tom lane