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