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Data Dictionary Views

Oracle maintains a data dictionary that records metadata about
  1. each object,
  2. the object owner,
  3. a definition, and
  4. related privileges.
For objects that require physical storage space of their own, Oracle will allocate space within a tablespace.
Tablespaces can be either dictionary managed or locally managed. In a dictionary-managed tablespace, space management is recorded in the data dictionary. In a locally managed tablespace (the default), Oracle maintains a bitmap in each datafile of the tablespace to track space availability.Only quotas are managed in the data dictionary, dramatically reducing the contention for data dictionary tables. Prior to performing a direct upgrade, you should analyze the data dictionary tables. During the upgrade process to Oracle Database 11g, the data dictionary will be analyzed if it has not been analyzed already, so performing this step in advance will aid the performance of the upgrade.

Aliases in View Creation

Internally, Oracle works with all column and table names in uppercase. This is how they are stored in its data dictionary, and this is how it always expects them to be. When aliases are typed to create a view, they should always exist without quotation marks around them. Putting double quotation marks around an alias can force the column name stored internally by Oracle to be in mixed case. If you do this, Oracle will not be able to find the column when you execute a select unless you enclose the column name within quotes during all your queries. Never use double quotation marks in creating aliases for a view.

Oracle Database SQL
1) Views that begin with USER list only those objects that you have created. For example, USER_INDEXES lists all the indexes that you created.

2) Views that begin with ALL contain information about all objects of that type that you either created yourself or to which you have access.

3) Views that begin with DBA can only be used by an Oracle user that has been given the DBA role. These views contain information about objects created by every user in the database. In addition, these views tend to contain more detailed information than similar views that begin with ALL or USER.