Following a question on OTN https://community.oracle.com/message/12801175 I did another test on redo and undo, just to prove that frequent COMMIT can be bad for performance. The results surprised me. I expected that row-by-row commit would be worse then a single commit at the end of a multi-row transaction, but I hadn't expected it to be this bad. As well as being much slower, both undo and redo volumes are vastly greater.
Buffer busy wait and related events can cripple performance of concurrent inserts. Bad in a single instance database, far worse in a RAC (think "gc buffer busy"). Often the problem is because of a primary key populated from a sequence. Reversing the index can fix this problem.
Concurrent inserts into a table will often result in crippling buffer busy wait problems as sessions serialize on access to the last block(s) of the table segment. Using hash clusters can remove the issue.
Many people are terrified of global indexes, one reason being that partition DDLs on the table will either render them unusable, or take forever as they are updated. Deferred global index maintenance solves this, and should be an important driver for the 12c upgrade.
Unified Audit is a major architectural change: fast, easy, and impossible for the DBA to bypass. On upgrade to Oracle 12c, you really should enable it. The earlier method that we all use is pretty awful.
The concept of an adaptive plan is that execution of a statement can start with one plan, and (during execution) switch to another. A pretty amazing capability. No need to run the statement to completion and try again: correct it in flight.
Why does the CBO get it wrong? Often because it has insufficient information. No matter how often you analyze your tables, if your queries use multi-column predicates, the CBO will mis-calculate the cardinalities. You have to understand your data, and create extended statistics to correlate the columns. How many people do this? Hardly any. Not a problem any more: release 12c can do this for you. If you configure it appropriately.