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Re: doubt about datum

From: Luca Ferrari <fluca1978(at)infinito(dot)it>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: doubt about datum
Date: 2007-08-06 14:57:29
Message-ID: 200708061657.29783.fluca1978@infinito.it (view raw or flat)
Thread:
Lists: pgsql-novice
On Tuesday 31 July 2007 Tom Lane's cat, walking on the keyboard, wrote:
> > Moreover why it does not need transaction
> > visibility as an ordinary tuple (xmin, cmin, cmax, xmax)?
>
> Datums are values.  '42'::int does not need visibility information;
> its value is always the same.

Maybe I understand something more, but I'm still in doubt about the tuple 
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?

Thanks,
Luca

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