Concurrency control questions 6.3.2 vs. 6.4

Hello:

I noticed the locking code in the backend/storage/lmgr directory has had a
lot of modifications between 6.3.2 vs. 6.4.  I know that Vadim is working
on changing the table-level locking scheme of 6.3.2 towards a
multi-version concurrency control scheme.  I'm wondering how much along
these modifications are -- it looks like there were changes made to the
existing locking scheme but no additional features were added.  This is
based on a very cursory look at the locking code in 6.4 (the locking code
is a lot more complicated than I had initially thought it was going to
be).

I'm curious as to how the multi-version scheme will be implemented.  Vadim
said that Postgres has a non-overwriting storage manager which can be
exploited for this concurrency control scheme.  I'm not sure I understand
him -- values that are updated in a table are written to the database in
such a fashion that the old value remains accessible?  This is
accomplished without a recovery log?

Also, there is some user-level locking code in the contrib directory by
Massimo that (if I am correct in my understanding of it), seems to be
providing row-level locking capabilities through query selects.  Is this
something that will be added to the Postgresql core at a future date?

Thanks in advance for any information you can provide.

--------------< LINUX: The choice of a GNU generation. >--------------
Steve Frampton  <3srf@qlink.queensu.ca>  http://qlink.queensu.ca/~3srf