SQL> explain plan for select count(1) from emp;

Explained.

SQL> select * from table(dbms_xplan.display('PLAN_TABLE',null,'ALL'));
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------
Plan hash value: 3381701820

-------------------------------------------------------------------
| Id  | Operation        | Name   | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |     1 |     1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |        |     1 |            |          |
|   2 |   INDEX FULL SCAN| EMP_PK |    15 |     1   (0)| 00:00:01 |
-------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

   1 - SEL$1
   2 - SEL$1 / EMP@SEL$1

Column Projection Information (identified by operation id):
-----------------------------------------------------------

   1 - (#keys=0) COUNT(*)[22]