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Multitenant : Overview of Container Databases (CDB) and Pluggable Databases (PDB)

Oracle 12c Release 1 (12.1) introduced the Multitenant option. This article provides a basic overview of the multitenant option, with links to more detailed articles on the functionality.

Overview

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).

Multitenant Overview

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.

Multitenant Create From Seed

Instead of creating a new PDB from the seed, you can clone an existing PDB.

Multitenant Clone

A more detailed description of creating and cloning PDBs can be found here.

Unpluging and Plugging in Pluggable Databases (PDBs)

One of the most powerful features of the multitenant option is the ability to unplug a PDB from a CDB and plug it back into another CDB.

Multitenant Unplug Plug

Not only does this allow databases to be moved easily, but it also paves the way for quick patching and upgrading to future versions. A PDB can be unplugged from a 12.1 CBD and plugged into a 12.2 CDB, effectively upgrading it in seconds. The unplug/plugin mechanism is described in more details here.

Conversion of a non-CDB database to a pluggable database involves getting a description the non-CDB database and using this to plug it into a CDB as a new PDB. This method is described here.

Views

The introduction of the multitenant option brings with it an extra layer of data dictionary views, allowing reporting across the root container and the pluggable databases (PDBs). Ignoring editions for the moment, prior releases had the following hierarchy.

DBA_ : All objects in the database.
|
--ALL_ : Objects accessible by the current user, including those owned by the current user.
  |
  --USER_ : Objects owned by the current user.

With Oracle 12c, an extra layer is added to the hierarchy.

CDB_ : All objects in the root container and all PDBs.
|
--DBA_ : All objects in the root container or PDB, depending on the current settings.
  |
  --ALL_ : Objects accessible by the current user, including those owned by the current user.
    |
    --USER_ : Objects owned by the current user.

The views are described in the Reference Manual.

Multitenant Articles

The following articles provide more detailed explanations of some of the concepts described in this article.

For more information see:

Hope this helps. Regards Tim...

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