Re: How is statement level read consistency implemented?

Pavan Deolasee, 22.04.2008 12:57:
> On Tue, Apr 22, 2008 at 4:11 PM, Thomas Kellerer <spam_eater@gmx.net> wrote:
>>
>>  I am interested in the technical details on how PG determines that a block
>> needs to be read from from "some other place than the data block" because
>> another transaction has updated the data block.
>>
> Postgres uses MVCC for transaction consistency. When a row is updated,
> a new copy of the row is created and the old version is marked for
> deletion (though its not removed immediately). The old readers
> continue to read from the old copy whereas new transactions see the
> new copy. This is all managed by XMIN and XMAX which are transaction
> ids of the inserting and deleting transactions respectively and
> control the visibility of the different versions.
>
> If you want to understand MVCC, please see this excellent
> documentation by Tom Lane:
> http://www.postgresql.org/files/developer/transactions.pdf

The xmin, xmax info is basically what I was looking for.

As far as I can tell (from the PDF and your quote) Postgres uses a very similar concept as Oracle. .
Each transaction has a uniqe number and each tuple contains the information for which transaction number it is visible.


Thanks for the quick answer.
Thomas