From the user and application perspective, the data dictionary in each container in a CDB is separate, as it would be in a non-CDB.
For example, the DBA_OBJECTS view in each PDB can show a different number of rows. This dictionary separation enables Oracle Database to manage the PDBs separately from each other and from the root.
In a CDB, the data dictionary metadata is split between the root and the PDBs. If USER$ table is in the PDB database then the data dictionary for this user data also resides in the PDB. Thus, the TAB$ table in the PDB has a row for the employees table and a row for the departments table.
The data dictionary views in the PDB and in the root contain different rows.
In container database:
SQL>select count(*) from tab$; 2226
In pluggable database:
SQL>select count(*) from tab$; 28167
Internally, Oracle-supplied objects such as data dictionary table definitions and PL/SQL packages are represented only in the root.
This architecture achieves two main goals within the CDB:
1) Reduction of duplication
For example, instead of storing the source code for the DBMS_ADVISOR PL/SQL package in every PDB, the CDB stores it only in CDB$ROOT, which saves disk space.
2) Ease of database upgrade
If the definition of a data dictionary table existed in every PDB, and if the definition were to change in a new release, then each PDB would need to be upgraded separately to capture the change. Storing the table definition only once in the root eliminates this problem.
The CDB uses an internal linking mechanism to separate data dictionary information. There are three kind of links used.
Metadata links
Oracle Database stores metadata about dictionary objects only in the CDB root. For example, the column definitions for the OBJ$ dictionary table, which underlies the DBA_OBJECTS data dictionary view, exist only in the root. the OBJ$ table in each PDB uses an internal mechanism called a metadata link to point to the definition of OBJ$ stored in the root.
The data corresponding to a metadata link resides in its PDB, not in the root. For example, if you create table mytable in hrpdb and add rows to it, then the rows are stored in the PDB data files.
Data links
In some cases, Oracle Database stores the data (not only metadata) for an object only once in the application root. An application PDB uses an internal mechanism called a data link to refer to the objects in the application root. The application PDB in which the data link was created also stores the data link description. A data link inherits the data type of the object to which it refers.
Extended data link
An extended data link is a hybrid of a data link and a metadata link. Like a data link, an extended data link refers to an object in an application root. However, the extended data link also refers to a corresponding object in the application PDB. Like a metadata link, the object in the application PDB inherits metadata from the corresponding object in the application root.
When queried in the application root, an extended data-linked object fetches rows only from the application root. However, when queried in an application PDB, an extended data-linked object fetches rows from both the application root and application PDB.
Oracle Database automatically creates and manages metadata and data links to CDB$ROOT. Users cannot add, modify, or remove these links.
- Oracle Multitenant DB 4 : Parameters/SGA/PGA management in CDB-PDB - July 18, 2021
- Oracle Multitenant DB 3 : Data Dictionary Architecture in CDB-PDB - March 20, 2021
- Oracle Multitenant DB 2 : Benefits of the Multitenant Architecture - March 19, 2021