INDEX RANGE SCAN will be skipped if first character in the string to be search will be wildcard

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

SQL> exec dbms_stats.gather_table_stats('SCOTT','EMP',cascade=>TRUE);

PL/SQL procedure successfully completed.

First of all we are testing all the index scan without using any wild card as a first character.

SQL> set autot traceonly;
SQL> select * from emp where ename LIKE 'S%OT%T%%';


Execution Plan
----------------------------------------------------------
Plan hash value: 3307655913

---------------------------------------------------------------------------------------------
| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |               |     7 |   273 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP           |     7 |   273 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EMP_ENAME_IDX |     7 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

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

   2 - access("ENAME" LIKE 'S%OT%T%%')
       filter("ENAME" LIKE 'S%OT%T%%')


Statistics
----------------------------------------------------------
        165  recursive calls
          0  db block gets
         30  consistent gets
          0  physical reads
          0  redo size
        826  bytes sent via SQL*Net to client
        381  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
          1  rows processed 

Now testing using first character a wild card character.

    
SQL> set autot traceonly
SQL>  select * from emp where ename LIKE '%S%OT%T%%';


Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    39 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP  |     1 |    39 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter("ENAME" LIKE '%S%OT%T%%')


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          8  consistent gets
          0  physical reads
          0  redo size
        822  bytes sent via SQL*Net to client
        381  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

It seems that whenever we are using a willcard ('%', '_') as a first character then it is skipping index scan becuase Wildchad is making the string to be search uncertain.
Let do one more experiment. Now we hide the wild card character with escape. like

SQL>  select * from emp where ename LIKE '/_S%OT%T%%' escape '/';

no rows selected


Execution Plan
----------------------------------------------------------
Plan hash value: 3307655913

---------------------------------------------------------------------------------------------
| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |               |     7 |   273 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP           |     7 |   273 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EMP_ENAME_IDX |     7 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

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

   2 - access("ENAME" LIKE '/_S%OT%T%%' ESCAPE '/')
       filter("ENAME" LIKE '/_S%OT%T%%' ESCAPE '/')


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          1  consistent gets
          0  physical reads
          0  redo size
        656  bytes sent via SQL*Net to client
        370  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed

SQL>