Explanation/Reference:
Explanation:

Example:
SQL> startup force
ORA-00824: cannot set SGA_TARGET or MEMORY_TARGET due to existing internal settings ORA-00848: STATISTICS_LEVEL cannot be set to BASIC with SGA_TARGET or
MEMORY_TARGET

Once DDL logging is turned on, every DDL command will be logged in the alert log file and also the log.xml file.
Note:
*By default Oracle database does not log any DDL operations performed by any user. The default settings for auditing only logs DML operations.
*Oracle 12c DDL Logging - ENABLE_DDL_LOGGING
The first method is by using the enabling a DDL logging feature built into the database. By default it is turned off and you can turn it on by setting the value of ENABLE_DDL_LOGGING initialization parameter to true.
*We can turn it on using the following command. The parameter is dynamic and you can turn it
on/off on the go.
SQL> alter system set ENABLE_DDL_LOGGING=true;
System altered.
Elapsed: 00:00:00.05
SQL>
Once it is turned on, every DDL command will be logged in the alert log file and also the log.xml file.

Explanation/Reference:
Explanation:
Oracle 11g/12c makes further use of SQL tuning sets with the SQL Performance Analyzer, which compares the performance of the statements in a tuning set before and after a database change. The database change can be as major or minor as you like, such as:
* (E) Database, operating system, or hardware upgrades.
* (A,C) Database, operating system, or hardware configuration changes.
* Database initialization parameter changes.
* Schema changes, such as adding indexes or materialized views.
* Refreshing optimizer statistics.
* Creating or changing SQL profiles.