* 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. The size of the seed PDB is only about 1 gigabyte and it takes only a few seconds on a typical machine to copy it.

Explanation/Reference:
Explanation:
* The environment setup for tablespace encryption is the same as that for transparent data encryption.
Before attempting to create an encrypted tablespace, a wallet must be created to hold the encryption key.
* Setting the tablespace master encryption key is a one-time activity. This creates the master encryption key for tablespace encryption. This key is stored in an external security module (Oracle wallet) and is used to encrypt the tablespace encryption keys.
* Before you can create an encrypted tablespace, the Oracle wallet containing the tablespace master encryption key must be open. The wallet must also be open before you can access data in an encrypted tablespace.
* Salt is a way to strengthen the security of encrypted data. It is a random string added to the data before it is encrypted, causing repetition of text in the clear to appear different when encrypted. Salt removes the one common method attackers use to steal data, namely, matching patterns of encrypted text.
* ALT | NO SALT By default the database appends a random string, called "salt," to the clear text of the column before encrypting it. This default behavior imposes some limitations on encrypted columns:
/If you specify SALT during column encryption, then the database does not compress the data in the encrypted column even if you specify table compression for the table. However, the database does compress data in unencrypted columns and encrypted columns without the SALT parameter.

Explanation/Reference:

* SHUTDOWN [ABORT | IMMEDIATE | NORMAL | TRANSACTIONAL [LOCAL]]
Shuts down a currently running Oracle Database instance, optionally closing and dismounting a
database. If the current database is a pluggable database, only the pluggable database is closed.
The consolidated instance continues to run.
Shutdown commands that wait for current calls to complete or users to disconnect such as
SHUTDOWN NORMAL and SHUTDOWN TRANSACTIONAL have a time limit that the
SHUTDOWN command will wait. If all events blocking the shutdown have not occurred within the
time limit, the shutdown command cancels with the following message:
ORA-01013: user requested cancel of current operation
* If logged into a CDB, shutdown closes the CDB instance.
To shutdown a CDB or non CDB, you must be connected to the CDB or non CDB instance that
you want to close, and then enter
SHUTDOWN
Database closed.
Database dismounted.
Oracle instance shut down.
To shutdown a PDB, you must log into the PDB to issue the SHUTDOWN command.
SHUTDOWN
Pluggable Database closed.
Note:
* Prerequisites for PDB Shutdown
When the current container is a pluggable database (PDB), the SHUTDOWN command can only be used if:
The current user has SYSDBA, SYSOPER, SYSBACKUP, or SYSDG system privilege.
The privilege is either commonly granted or locally granted in the PDB.
The current user exercises the privilege using AS SYSDBA, AS SYSOPER, AS SYSBACKUP, or AS SYSDG at connect time.
To close a PDB, the PDB must be open.