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.

Oracle 如何使用数据字典
1. 数据字典基表中的数据是 Oracle 正常工作所必须的
2. 在数据库运行期间，Oracle 通过数据字典内的信息来确定方案对象是否存在，以及用户是否具备相应的权
限。Oracle会持续地更新数据字典，以便反映数据库在结构，数据，授权等方面的变化