[HACKERS] Why there is a union in HeapTupleHeaderData struct

[Soroosh Sardari]

Dear Hackers

In fix part oh HeapTuple, there is a union that is named t_choice,
union
    {
        HeapTupleFields t_heap;
        DatumTupleFields t_datum;
    }            t_choice;

I can't find out why we need t_datum, actually there is no comment about
DatumTupleFields.

Regards
Soroosh