Just a nit, but Oracle implements MVCC. 90% of the databases out there do.
Sorry, I spoke imprecisely. What I meant was the difference in how the rows are stored internally .... in Oracle, the main tablespace contains only the newest version of a row, which is (where possible) updated in place - queries in a transaction that can still "see" an older version have to pull it from the UNDO tablespace (rollback segments in Oracle 8 and older).
In Postgres, all versions of all rows are in the main table, and have validity ranges associated with them ("this version of this row existed between transaction ids x and y"). Once a version goes out of scope, it has to be garbage collected by the vacuuming process so the space can be re-used.
In general, this means Oracle is faster *if* you're only doing lots of small transactions (consider how these different models handle an update to a single field in a single row) but it is more sensitive to the scale of transactions .... doing a really big transaction against a database with an OLTP workload can upset Oracle's digestion as it causes a lot of UNDO lookups, PG's performance is a lot more predictable in this regard.
Both models have benefits and drawbacks ... when designing a schema for performance it's important to understand these differences.
Yes, absolutely. It's not unusual to see the UNDO tablespace increase in size by several gigs for a large bulk load.
Speaking of rollback segments I'm assuming that since all storage for non-visible row versions is in the main table that PostgreSQL has no equivalent for an ORA-01555.
