How is statement level read consistency implemented?

Hi,

in my office we were discussing the various ways statement level read
consistency is implemented in different databases, namely Oracle and
Postgres.

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.

I'm referring to the scenario that Tom Kyte describes here:
www.oracle.com/technology/oramag/oracle/05-nov/o65asktom.html
and how PG would detect that row 342,023 has been modified by a different
transaction (and thus reads the correct value, unlike "the other
database")

Oracle assigns a SCN (system change number) to each transaction, each
block contains the SCN for which it is valid, any number higher than the
current SCN indicates that the block has to be taken from the rollback
segment.

How is this "test" implemented in Postgres? 
Does it have a similar concept (SCN) or does WAL imply a completely
different way?


Thanks in advance
Thomas



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