What is Data dictionary ?
Data Dictionary is basically a repository which contains data about the database such as information about tables , column names,their length , datatypes, attributes and other database structures etc. Also we know that data about the data or the structure containing other data is called metadata, In short we can say that data dictionary is a central repository that contains oracle metadata. When we query data dictionary , we actually query metadata.
Architecture of Data Dictionary :
Data Dictionary is basically composed of following two parts :
- Base Tables.
- User Accessible Views
Base Tables :
As discussed above Data Dictionary is basically a repository which contains data about the database. This data is actually stored in the Base tables. The data in Base Tables is in encrypted form and is not accessible to the users. The data of the base tables is only used by Oracle.
User Accessible Views :
Since the users do not have access to the Base Tables , in order to access the data present in Base Tables , User Accessible Views are used. They contain the information present in the Base Tables in the decoded and useful form and are accessible to the users. They breakdown the information present in the Base Tables in more useful form by combining them through joins and Where Clause.
Time of creation : Data dictionary is created at the time of oracle database creation.
Owner : Data Dictionary is owned by SYS user .
Location : It is basically located in the SYSTEM TABLESPACE of that Database.
Accessibility : The Data Dictionary is only Read Only and only SELECT Statement can be issued to query its tables and views.