An important part of an Oracle database is its data dictionary, which is a read-only set of tables that provides administrative metadata about the database. A data dictionary contains information such as the following:
- The definitions of every schema object in the database, including default values for columns and integrity constraint information.
- The amount of space allocated for and currently used by the schema objects
- The names of Oracle Database users, privileges and roles granted to users, and auditing information related to users
The data dictionary is a central part of data management for every Oracle database. For example, the database performs the following actions:
- Accesses the data dictionary to find information about users, schema objects, and storage structure
- Modifies the data dictionary every time that a DDL statement is issued
Contents of the Data Dictionary
The data dictionary consists of the following:
The underlying tables that store information about the associated database. Only Oracle should write to and read these tables. Users rarely access them directly because they are normalized, and most of the data is stored in a cryptic format.
The views that summarize and display the information stored in the base tables of the data dictionary. These views decode the base table data into useful information, such as user or table names, using joins and WHERE clauses to simplify the information. Most users are given access to the views rather than the base tables.
Related Post:- What Is View?
Storage of the Data Dictionary
The data dictionary base tables are the first objects created in any Oracle database. All data dictionary tables and views for a database are stored in the
SYSTEM tablespace. Because the SYSTEM tablespace is always online when the database is open, the data dictionary is always available when the database is open.How Oracle Uses the Data Dictionary
Data in the base tables of the data dictionary is necessary for Oracle to function. Therefore, only Oracle should write or change data dictionary information. Oracle provides scripts to modify the data dictionary tables when a database is upgraded or downgraded. During database operation, Oracle reads the data dictionary to ascertain that schema objects exist and that users have proper access to them. Oracle also updates the data dictionary continuously to reflect changes in database structures, auditing, grants, and data.
For example, if user Kathy creates a table named parts, then new rows are added to the data dictionary that reflect the new table, columns, segment, extents, and the privileges that Kathy has on the table. This new information is then visible the next time the dictionary views are queried.
Dictionary Cache is place in Shared Pool which contains Data Dictionary. Oracle frequently requires Data Dictionary. Most parts of Data Dictionary are cached into Dictionary Cache. Oracle utilizes Dictionary Cache information in query parsing. Dictionary cache is also called Row Cache as data inside Dictionary Cache is maintained into rows instead of buffer
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