Re: Why there is a union in HeapTupleHeaderData struct

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On 20-May-2013, at 18:14, Amit Langote <amitlangote09(at)gmail(dot)com> wrote:

> Wonder though if this question is better asked in pgsql-novice?
>
> On Mon, May 20, 2013 at 9:23 PM, Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
>> 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
>>>>>
>>>>>

I don't think so, as this involves internal structures.Let's keep it here.

Regards,

Atri