Re: postgresql's MVCC implementation

Tom Lane-2 wrote:
>
> If you want that to fail, use a SELECT FOR UPDATE at steps 3/4.
>
> My interpretation of MVCC is that the above example isn't even
> meaningful, because it assumes that "writing into Y" is an overwrite,
> which it is not in Postgres --- that is, if T2 reads Y again, it'll
> get the same value as before.
>

Hi Tom,

Thanks for your reply. I think what I'd like to know is the exact meaning
of MVCC as implemented in PostgreSQL. It seems that a transaction
(with isolation set to serializable) will always read the values as if they
were when the transaction started.

If it is the case, why? Is MVCC not well defined? Could say Oracle or MS
SQL implement it differently?

-----
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Kent Tong
Wicket tutorials freely available at http://www.agileskills2.org/EWDW
Axis2 tutorials freely available at http://www.agileskills2.org/DWSAA
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