Data Dictionary数据字典oracle(三)
ata dictionary to ascertain that schema objects exist and that users have proper access to them. Oracle Database also updates the data dictionary continuously to reflect changes in database structures, auditing, grants, and data.
For example, if user hr creates a table named interns, then new rows are added to the data dictionary that reflect the new table, columns, segment, extents, and the privileges that hr has on the table. This new information is visible the next time the dictionary views are queried.
Public Synonyms for Data Dictionary Views
Oracle Database creates public synonyms for many data dictionary views so users can access them conveniently. The security administrator can also create additional public synonyms for schema objects that are used systemwide. Users should avoid naming their own schema objects with the same names as those used for public synonyms.
Cache the Data Dictionary for Fast Access
Much of the data dictionary information is in the data dictionary cache because the database constantly requires the information to validate user access and verify the state of schema objects. Parsing information is typically kept in the caches. The COMMENTS columns describing the tables and their columns are not cached in the dictionary cache, but may be cached in the database buffer cache.
Other Programs and the Data Dictionary
Data DictionaryOther Oracle Database products can reference existing views and create additional data dictionary tables or views of their own. Application developers who write programs that refer to the data dictionary should refer to the public synonyms rather than the underlying tables. Synonyms are less likely to change between releases.