[tomiko_nakamura]

PostgreSQL does not have in-place update in the first place, on every UPDATE it does a DELETE+INSERT. So "partial update" does not make much sense, because you'll create a new copy of the row anyway.

[NickNameNick]

That's not quite how PostgreSQL's Multi-value-Concurrency system works.

[tomiko_nakamura]

MVCC means "Multiversion Concurrency Control" and yes, that's pretty much exactly how it works (creating copies of the rows, so that different snapshots see different rows).

In practice it's more complicated of course. A nice intro to MVCC is available here: http://momjian.us/main/writings/pgsql/mvcc.pdf (and in the Bruce's other talks http://momjian.us/main/presentations/internals.html).

Or maybe you mean something else by Multi-value-Concurrency. In that case please elaborate.