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The SQL standard defines four levels of isolation in terms of the phenomena that a transaction running at a particular isolation level is permitted to experience
see this link :)
1. - refer to doc http://download.oracle.com/docs/cd/B28359_01/server.111/b28318/consist.htm -very much clear
2 - vpd demo - http://www.oracle.com/technetwork/database/security/index-088277.html - hope you understand
- Pavan Kumar N
1. Neither SQL standard nor Oracle has an "optimistic transaction isolation level". There are only optimistic locking and pessimistic locking strategies.
2. From 11.2 Concepts Guide:
Virtual Private Database (VPD) enables you to enforce security at the row and column level. A security policy establishes methods for protecting a database from accidental or malicious destruction of data or damage to the database infrastructure.
VPD is useful when security protections such as privileges and roles are not sufficiently fine-grained. For example, you can allow all users to access the employees table, but create security policies to restrict access to employees in the same department as the user.
Essentially, the database adds a dynamic WHERE clause to a SQL statement issued against the table, view, or synonym to which an Oracle VPD security policy was applied. The WHERE clause allows only users whose credentials pass the security policy to access the protected data.