Re: Why there is a union in HeapTupleHeaderData struct

Hello,

I think a more appropriate question to be asked here would be at what
point (in the life of a typical tuple), does a tuple's header contain
t_datum or otherwise, which I would also like to be answered.

On Mon, May 20, 2013 at 6:06 PM, Soroosh Sardari
<soroosh(dot)sardari(at)gmail(dot)com> wrote:
> 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
>
>

--
Amit Langote