Re: doubt about datum

Luca Ferrari <fluca1978@infinito.it> writes:
> 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?

The t_datum fields are for use in an in-memory tuple that represents a
value of a composite type.  Being a Datum, it needs a length word and
some other stuff that are not needed in a row-on-disk; and it does not
need visibility info, which is what the t_heap fields are.

            regards, tom lane