Oracle data dictionary
In a database, all of the objects that belong to a user form what is called a user schema.
The main types of objects are tables, indexes, views, synonyms, sequences, program units stored in the database (procedures, functions, packages, triggers).
Only tables and indexes correspond to segments of data. Other objects only have a definition stored in the data dictionary.
A user schema is created and managed by it, while the data dictionary is created and managed by Oracle.
The data dictionary is a repository which belongs to the SYS super user and which stores the tables and views necessary for the proper functioning of the Oracle database. The DBA_TABLES view and the corresponding synonym USER_TABLES is an example of a table and dictionary view.
For example, if a user USER1 wants to display the structure of a TABLE1 table that he owns, he issues the "DESC TABLE1" command. The result of this command is obtained from the data dictionary.
This is because the TABLE1 table stores data and is owned by user USER1, while the structure of TABLE1 is stored elsewhere, in dictionary tables that are owned by user SYS. This information about the data forming the structure of TABLE1 is called metadata.
Dictionary tables are not directly accessible but dictionary views allow you to query them. In a session of the user USER1, USER_TABLES (synonymous with the DBA_TABLES view) is used to list the structure of all the tables of USER1.
In summary, the data dictionary stores metadata on tables, columns of tables, constraints, indexes, etc.
It can also store:
user objects such as views, synonyms, sequences, procedures, functions, packages, triggers, etc.
the names of the users, their privileges and roles… etc.
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