* In most cases, ADDM output should be the first place that a DBA looks when notified of a performance problem.
* Performance degradation of the database occurs when your database was performing optimally in the past, such as 6 months ago, but has gradually degraded to a point where it becomes noticeable to the users. The Automatic Workload Repository (AWR) Compare Periods report enables you to compare database performance between two periods of time.
While an AWR report shows AWR data between two snapshots (or two points in time), the AWR Compare Periods report shows the difference between two periods (or two AWR reports with a total of four snapshots). Using the AWR Compare Periods report helps you to identify detailed performance attributes and configuration settings that differ between two time periods.
Reference: Resolving Performance Degradation Over Time

* Data Recovery Advisor can diagnose failures such as the following:
/ (B) Components such as datafiles and control files that are not accessible because they do not
exist, do not have the correct access permissions, have been taken offline, and so on
/ (A, E) Physical corruptions such as block checksum failures and invalid block header field values
/ Inconsistencies such as a datafile that is older than other database files
/ I/O failures such as hardware errors, operating system driver failures, and exceeding operating system resource limits (for example, the number of open files)
* The Data Recovery Advisor automatically diagnoses corruption or loss of persistent data on disk, determines the appropriate repair options, and executes repairs at the user's request. This reduces the complexity of recovery process, thereby reducing the Mean Time To Recover (MTTR).

Explanation/Reference:
Explanation:
If you run multiple AUDIT statements on the same unified audit policy but specify different EXCEPT users, then Oracle Database uses the last exception user list, not any of the users from the preceding lists. This means the effect of the earlier AUDIT POLICY ... EXCEPT statements are overridden by the latest AUDIT POLICY ... EXCEPT statement.
Note:
* The ORA_DATABASE_PARAMETER policy audits commonly used Oracle Database parameter settings.
By default, this policy is not enabled.
* You can use the keyword ALL to audit all actions. The following example shows how to audit all actions on the HR.EMPLOYEES table, except actions by user pmulligan.
Example Auditing All Actions on a Table
CREATE AUDIT POLICY all_actions_on_hr_emp_pol
ACTIONS ALL ON HR.EMPLOYEES;
AUDIT POLICY all_actions_on_hr_emp_pol EXCEPT pmulligan;

Explanation/Reference:
Explanation:
A, Not D: In a CDB, PUBLIC is a common role. In a PDB, privileges granted locally to PUBLIC enable all local and common users to exercise these privileges in this PDB only.
C: A user can only perform common operations on a common role, for example, granting privileges commonly to the role, when the following criteria are met:
The user is a common user whose current container is root.
The user has the SET CONTAINER privilege granted commonly, which means that the privilege applies in all containers.
The user has privilege controlling the ability to perform the specified operation, and this privilege has been granted commonly
Note:
* Every privilege and role granted to Oracle-supplied users and roles is granted commonly except for system privileges granted to PUBLIC, which are granted locally.