MYDBA@ORCL >
MYDBA@ORCL > start explain_function;
MYDBA@ORCL >
MYDBA@ORCL > create table test(a) as select rownum from all_objects where rownum <= 10;

Table created.

MYDBA@ORCL >
MYDBA@ORCL > create or replace function f return number
  2  as
  3          l_count number;
  4  begin
  5          select count(*) into l_count from test;
  6          return l_count;
  7  end;
  8  /

Function created.

MYDBA@ORCL > show errors
No errors.
MYDBA@ORCL >
MYDBA@ORCL > select f from dual;

         F
----------
        10

1 row selected.

MYDBA@ORCL >
MYDBA@ORCL > explain plan for select f from dual;

Explained.

MYDBA@ORCL >
MYDBA@ORCL > select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------

Plan hash value: 1388734953

-------------------------------------------------------------------------
| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     1 |       |     2   (0)| 00:00:01 |
|   1 |  FAST DUAL       |      |     1 |       |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------

8 rows selected.

MYDBA@ORCL >
MYDBA@ORCL > drop function f;

Function dropped.

MYDBA@ORCL > drop table test;

Table dropped.

MYDBA@ORCL >
MYDBA@ORCL > set echo off;
MYDBA@ORCL >