Oracle Database Discussions

Hi,

I have 2 short questions:

1. I know Oracle has these 3 transaction isolation levels (a) Read commited, (b) Serializable (c) Read Only. But I have heard about a 4th one, called something like "Optimistic isolation level", I heard about it, and thought I would search it in oracle documentation on internet. But now I have searched a lot, almost a whole day, but I couldn't find any such thing. All I found was optimistic locking. Can anybody tell me what is meant by "Optimistic transaction isolation level" ??? and what it does?

2. Can anybody tell me what is "Virtual Private Database" in Oracle, I searched on net and found some documentation, but they are very lengthy and are not very clear. So, Can anybody tell me in short words, what is this?

Thanks in Advance.