Explanation/Reference:
Explanation:
Note:
* DBMS_REDACT.FULL completely redacts the column data.
* DBMS_REDACT.NONE applies no redaction on the column data. Use this function for development testing purposes. LOB columns are not supported.
* The DBMS_REDACT package provides an interface to Oracle Data Redaction, which enables you to mask (redact) data that is returned from queries issued by low-privileged users or an application.
* If you create a view chain (that is, a view based on another view), then the Data Redaction policy also applies throughout this view chain. The policies remain in effect all of the way up through this view chain, but if another policy is created for one of these views, then for the columns affected in the subsequent views, this new policy takes precedence.

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

* The CREATE DATABASE ... ENABLE PLUGGABLE DATABASE SQL statement creates a new CDB. If you do not specify the ENABLE PLUGGABLE DATABASE clause, then the newly created database is a non-CDB and can never contain PDBs.
Along with the root (CDB$ROOT), Oracle Database automatically creates a seed PDB (PDB$SEED). The following graphic shows a newly created CDB: * Creating a PDB Rather than constructing the data dictionary tables that define an empty PDB from scratch, and then populating its Obj$ and Dependency$ tables, the empty PDB is created when the CDB is created. (Here, we use empty to mean containing no customer-created artifacts.) It is referred to as the seed PDB and has the name PDB$Seed. Every CDB non-negotiably contains a seed PDB; it is non-negotiably always open in read-only mode. This has no conceptual significance; rather, it is just an optimization device. The create PDB operation is implemented as a special case of the clone PDB operation.

C:During the first execution of a SQL statement, an execution plan is
generated as usual.
D:if multi-column statistics are not present for the relevant combination of columns, the
optimizer can fall back on cardinality feedback.
(not B)*Cardinality feedback. This feature, enabled by default in 11.2, is intended to
improve plans for repeated executions.
optimizer_dynamic_sampling
optimizer_features_enable
*dynamic sampling or multi-column statistics allow the optimizer to more accurately
estimate selectivity of conjunctive predicates.
Note:
*OPTIMIZER_DYNAMIC_SAMPLING controls the level of dynamic sampling performed by
the optimizer.
Range of values.0 to 10
*Cardinality feedback was introduced in Oracle Database 11gR2. The purpose of this
feature is to automatically improve plans for queries that are executed repeatedly, for which
the optimizer does not estimate cardinalities in the plan properly. The optimizer may
misestimate cardinalities for a variety of reasons, such as missing or inaccurate statistics,
or complex predicates. Whatever the reason for the misestimate, cardinality feedback may
be able to help.

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.