https://blogs.oracle.com/optimizer/entry/cardinality_feedback

http://docs.oracle.com/database/121/ADMIN/cdb_pdb_admin.htm#ADMIN14017

* The SGA_TARGET parameter can be dynamically increased up to the value specified for the SGA_MAX_SIZE parameter, and it can also be reduced.
*Example:
For example, suppose you have an environment with the following configuration:
SGA_MAX_SIZE = 1024M SGA_TARGET = 512M DB_8K_CACHE_SIZE = 128M In this example, the value of SGA_TARGET can be resized up to 1024M and can also be reduced until one or more of the automatically sized components reaches its minimum size. The exact value depends on environmental factors such as the number of CPUs on the system. However, the value of DB_8K_CACHE_SIZE remains fixed at all times at 128M
*DB_8K_CACHE_SIZE Size of cache for 8K buffers
*For example, consider this configuration:
SGA_TARGET = 512M DB_8K_CACHE_SIZE = 128M In this example, increasing DB_8K_CACHE_SIZE by 16 M to 144M means that the 16M is taken away from the automatically sized components. Likewise, reducing DB_8K_CACHE_SIZE by 16M to 112M means that the 16M is given to the automatically sized components.

* 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.

* SQL Access Advisor is primarily responsible for making schema modification recommendations, such as adding or dropping indexes and materialized views. SQL Tuning Advisor makes other types of recommendations, such as creating SQL profiles and restructuring SQL statements.
* The query optimizer can also help you tune SQL statements. By using SQL Tuning Advisor and SQL Access Advisor, you can invoke the query optimizer in advisory mode to examine a SQL statement or set of statements and determine how to improve their efficiency. SQL Tuning Advisor and SQL Access Advisor can make various recommendations, such as creating SQL profiles, restructuring SQL statements, creating additional indexes or materialized views, and refreshing optimizer statistics.
Note:
* Decision support system (DSS) workload
* The library cache is a shared pool memory structure that stores executable SQL and PL/SQL code. This cache contains the shared SQL and PL/SQL areas and control structures such as locks and library cache handles.