Re: Why there is a union in HeapTupleHeaderData struct

Thanks,

If a tuple constructed in memory we don't need t_heap. I have another
question,
How make an in-memory tuple?

On Mon, May 20, 2013 at 12:46 PM, Amit Langote <amitlangote09(at)gmail(dot)com>wrote:

> Hello,
>
> I think the comment just above the HeapTupleFields struct definition
> has the related details.
>
> /*
> * Heap tuple header. To avoid wasting space, the fields should be
> * laid out in such a way as to avoid structure padding.
> *
> * Datums of composite types (row types) share the same general structure
> * as on-disk tuples, so that the same routines can be used to build and
> * examine them. However the requirements are slightly different: a Datum
> * does not need any transaction visibility information, and it does need
> * a length word and some embedded type information. We can achieve this
> * by overlaying the xmin/cmin/xmax/cmax/xvac fields of a heap tuple
> * with the fields needed in the Datum case. Typically, all tuples built
> * in-memory will be initialized with the Datum fields; but when a tuple is
> * about to be inserted in a table, the transaction fields will be filled,
> * overwriting the datum fields.
>
>
> especially the last line points as to what roles each of them plays,
> though, I would like to hear more about additional details from others
> who might reply.
>
>
>
> On Mon, May 20, 2013 at 4:28 PM, Soroosh Sardari
> <soroosh(dot)sardari(at)gmail(dot)com> wrote:
> > 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
> >
> >
>
>
>
> --
> Amit Langote
>