The data dictionary comprises a set of tables and views that map the structure of the database.
Oracle databases store information here about the logical and physical structure of the database. The data dictionary contains information such as:
- user information, such as user privileges
- integrity constraints defined for tables in the database
- names and datatypes of all columns in database tables
- information on space allocated and used for schema objects
The Oracle instance frequently accesses the data dictionary in order to parse SQL statements. The operation of Oracle depends on ready access to the data dictionary: performance bottlenecks in the data dictionary affect all Oracle users. Because of this, database administrators should make sure that the data dictionary cache[10] has sufficient capacity to cache this data. Without enough memory for the data-dictionary cache, users see a severe performance degradation. Allocating sufficient memory to the shared pool where the data dictionary cache resides precludes these particular performance problems.
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