Обсуждение: How are NULLS stored on disk for PG 7.4?

Hi

How are column files in a table that have been assigned null stored on disk in Postgresql 7.4?

Cheers
Arn
 

Arnold Mavromatis wrote:
> Hi
>
> How are column files in a table that have been assigned null stored on disk
> in Postgresql 7.4?

Each row has a bitmap showing all the NULL values on that row.

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Le mar 13/01/2004 à 00:43, Bruce Momjian a écrit :
> Arnold Mavromatis wrote:
> > Hi
> >
> > How are column files in a table that have been assigned null stored on disk
> > in Postgresql 7.4?
>
> Each row has a bitmap showing all the NULL values on that row.

Is it the same for PG 7.3?

Yannick

On Wed, 14 Jan 2004, Yannick Warnier wrote:

> Le mar 13/01/2004 à 00:43, Bruce Momjian a écrit :
> > Arnold Mavromatis wrote:
> > > Hi
> > >
> > > How are column files in a table that have been assigned null stored on disk
> > > in Postgresql 7.4?
> >
> > Each row has a bitmap showing all the NULL values on that row.
>
> Is it the same for PG 7.3?

Yes, Postgresql has done this for quite some time.

> On Wed, 14 Jan 2004, Yannick Warnier wrote:
>
>> Le mar 13/01/2004 à 00:43, Bruce Momjian a écrit :
>> > Arnold Mavromatis wrote:
>> > > Hi
>> > >
>> > > How are column files in a table that have been assigned null stored
>> on disk
>> > > in Postgresql 7.4?
>> >
>> > Each row has a bitmap showing all the NULL values on that row.
>>
>> Is it the same for PG 7.3?
>
> Yes, Postgresql has done this for quite some time.
>
My understanding is that Oracle has a field showing the size of the column
prior to each column. A column size of of zero means that the column is
null.

I assume that this is why Oracle is unable to make a distinction between a
null and a zero length string but that PostgreSQL can?

Vincent

Yannick Warnier wrote:

> Le mar 13/01/2004 à 00:43, Bruce Momjian a écrit :
>> Arnold Mavromatis wrote:
>> > Hi
>> >
>> > How are column files in a table that have been assigned null stored on disk
>> > in Postgresql 7.4?
>>
>> Each row has a bitmap showing all the NULL values on that row.
>
> Is it the same for PG 7.3?

It has always been that way. But the bitmap is only there if there is at
least one NULL value. One bit in the tuple headers flags tells if there
is a bitmap or not.


Jan

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