Container Database
& Pluggable Databases:
The multitenant option represents one of the biggest architectural
changes in the history of the Oracle database. The option introduced the
concepts of the Container Database (CDB) and Pluggable Database (PDB).
- Container Databases (CDB) : This seems very similar to a conventional Oracle database, as it contains most of the working parts you will be already familiar with (controlfiles, datafiles, undo, tempfiles, redo logs etc.). It also houses the data dictionary for those objects that are owned by the root container and those that are visible to all PDBs.
- Pluggable Database (PDB) : Since the CDB contains most
of the working parts for the database, the PDB only needs to contain
information specific to itself. It does not need to worry about
controlfiles, redo logs and undo etc. Instead it is just made up of
datafiles and tempfiles to handle it's own objects. This includes it's own
data dictionary, containing information about only those objects that are specific
to the PDB.
This split of the data dictionary between common objects, in the root
container, and PDB-specific objects, in the PDB's data dictionary, is very
important, because this separation is what gives the multitenant option its
flexibility. From the perspective of the PDB, the data dictionary is the union
of the root and PDB data dictionaries, so internally the PDB feels very much
like a normal Oracle database. For example, the DBA_% and ALL_% views within the PDB appears the
same as any non-CDB database.
Creating Pluggable Databases (PDBs)
Since the bulk of the working parts are already present in the root
container, creating a new PDB is a comparatively quick and simple task. When
creating a completely new PDP, the PDB is created as a copy of a seed PDB, so
it only takes as long as the files take to copy.
Instead of creating a new PDB from the seed, you can clone an existing
PDB.
It is also possible to create clones in a remote CDB.