[oracle@oradb SQL]$ sqlplus sys@pdborcl AS sysdba

SQL*Plus: Release 12.1.0.1.0 Production on Fri Jul 19 18:39:23 2013

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Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> explain plan for
select --+ rule
   A.name
  ,C.blocks
  ,C.extents
from sys.obj$ A
    ,sys.tab$ B
    ,sys.seg$ C
where A.owner# = (select user# from sys.user$ where name = user)
  and B.obj#   = A.obj#
  and C.file#  = B.file#
  and C.block# = B.block#;

select * from table(dbms_xplan.display);
  2    3    4    5    6    7    8    9   10   11   12  
Explained.

SQL> SQL> 
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2880203025

--------------------------------------------------
| Id  | Operation                      | Name    |
--------------------------------------------------
|   0 | SELECT STATEMENT               |         |
|   1 |  NESTED LOOPS                  |         |
|   2 |   NESTED LOOPS                 |         |
|   3 |    TABLE ACCESS FULL           | SEG$    |
|*  4 |    INDEX RANGE SCAN            | I_OBJ2  |
|   5 |     TABLE ACCESS BY INDEX ROWID| USER$   |

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|*  6 |      INDEX UNIQUE SCAN         | I_USER1 |
|*  7 |   TABLE ACCESS CLUSTER         | TAB$    |
|*  8 |    INDEX UNIQUE SCAN           | I_OBJ#  |
--------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("A"."OWNER#"= (SELECT "USER#" FROM "SYS"."USER$" "USER$"
              WHERE "NAME"=USER@!))
   6 - access("NAME"=USER@!)

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
   7 - filter("C"."BLOCK#"="B"."BLOCK#" AND "C"."FILE#"="B"."FILE#")
   8 - access("B"."OBJ#"="A"."OBJ#")

Note
-----
   - rule based optimizer used (consider using cbo)

28 rows selected.