Luca Ferrari <fluca1978(at)infinito(dot)it> writes:
> I'm new to postgresql programming and I'd like to understand what a datum is
A value of any data type -- which particular type is not said by the
datum itself, but we always have context information to tell us that.
> 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.
regards, tom lane