A - false; data dictionary views are based only on dictionary base tables, not on DBA-defined tables
B - false; data dictionary is created and maintained by the Oracle database server
C - true; Oracle Database Workshop: "Generally, each view is a subset of the higher-privileged view set, row-wise and column wise."
D - true; SELECT * FROM dual; or SELECT * FROM ts$;
E - false; owner is SYS, tablespace is SYSTEM
F - true; usernames are stored in USER$ base table, they are accessible typically with DBA_USERS dictionary view
A is wrong. "DBA_defined tables" does not fit here. Dictionary views are based on system base tables only.
B is wrong. Internally created and managed by the Oracle server.
C is correct. They all have to reference the same base tables, they only display filtered rowsets from these tables.
D is correct. You can query these tables, however not recommended as data is stored in a cryptic format. Consider logging in as the SYS user, since you "own" the tables you can select from them. And as DBA you can grant select on any table in any schema to any user. This is a permission (ownership) issue and not a data format issue.
E is wrong. Owned by the SYS user, stored in the SYSTEM tablespace.
F is correct. All user's usernames are stored in the dictionary and are queries via DBA_USERS.
GUYS, the correcly answer are ACDF
A- User-Accessible Views
The views that summarize and display the information stored in the base tables of the data dictionary. These views decode the base table data into useful information, such as user or table names, using joins and WHERE clauses to simplify the information. Most users are given access to the views rather than the base tables.
https://docs.oracle.com/cd/B19306_01/server.102/b14220/datadict.htm
D is wrong. Base Tables
The underlying tables that store information about the associated database. Only Oracle should write to and read these tables. Users rarely access them directly because they are normalized, and most of the data is stored in a cryptic format.
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