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The data dictionary of Oracle

Oracle's data dictionary provides information that Oracle needs to perform its tasks. This information consists of definition, allocated and used storage size for database objects, default column values, integrity constraints, names of and privileges granted to users, auditing information and more.
The datadictionary is stored in a few tables owned by SYS (the so called dictionary base tables. Their content is exposed through the static dictionary views. These views and tables should not be written to, only selected. The base tables are stored in the system tablespace (which is always available when the database is open).
The data dictionary is updated when a DDL statement is executed.
Since the data in the data dictionary is fundamental to Oracle's functioning, frequently accessed parts of the data dictionary is stored in the dictionary cache in the SGA.