postgresql's MVCC implementation

Hi,

I read a description of MVCC in http://www.cs.ust.hk/~dimitris/CS530/L24.ppt
and
found that this isn't exactly what is implemented in PostgreSQL. For
example,
for a sequence of operations like:

1: T1 sets isolation to serializable & begins a transaction
2: T2 sets isolation to serializable & begins a transaction
3: T1 reads X into v1
4: T2 reads Y into v2
5: T1 writes v1 into Y
6: T2 writes v2 into X
7: T1 commits
8: T2 commits

Obviously, this sequence is also not a serializable execution. However, it
is allowed by
PostgreSQL. Moreover, according to the MVCC reference above, step 5 should
really
fail because the read timestamp of Y is that of T2, which is greater than
that of T1.

I understand that PostgreSQL doesn't implement predictive locking, but this
example
doesn't involve any phantom at all. It is plain multiversion timestamp
concurency control.

Any idea? Thanks in advance!

-----
--
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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