Re: doubt about datum

On Tuesday 31 July 2007 Tom Lane's cat, walking on the keyboard, wrote:
> > Moreover why it does not need transaction
> > visibility as an ordinary tuple (xmin, cmin, cmax, xmax)?
>
> Datums are values.  '42'::int does not need visibility information;
> its value is always the same.

Maybe I understand something more, but I'm still in doubt about the tuple
header. In the htup.h file you find:

typedef struct HeapTupleHeaderData
{
    union
    {
        HeapTupleFields t_heap;
        DatumTupleFields t_datum;
    }            t_choice;
        ....
}

maybe I cannot get the meaning of the above t_datum in the tuple header, can
anyone explain me that?

Thanks,
Luca