I am thinking that optimization must be specific to 11g R2, although I am not sure that I recall testing it on 11g R2. A brief test case table:

CREATE TABLE T1(
  C1 NUMBER,
  C2 VARCHAR2(200),
  PRIMARY KEY(C1));
 
INSERT INTO
  T1
SELECT
  ROWNUM C1,
  LPAD('A',200,'A') C2
FROM
  DUAL
CONNECT BY
  LEVEL<=1000000;
 
COMMIT;
 
EXEC DBMS_STATS.GATHER_TABLE_STATS(OWNNAME=>USER,TABNAME=>'T1',CASCADE=>TRUE)

Test case script:

SELECT
  C1,
  SUBSTR(C2,1,10) C2
FROM
  T1
WHERE
  C1=10000;
 
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'TYPICAL'));
 
SELECT
  C1,
  SUBSTR(C2,1,10) C2
FROM
  T1
WHERE
  NVL(C1,0)=10000;
 
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'TYPICAL'));
 
SELECT /*+ INDEX(T1) */
  C1,
  SUBSTR(C2,1,10) C2
FROM
  T1
WHERE
  NVL(C1,0)=10000;
 
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'TYPICAL'));

This is the output that I receive on Oracle Database 10.2.0.2 (very similar results were seen on 11.1.0.7):

SQL_ID  gywbrs9kx94v7, child number 0
-------------------------------------
SELECT   C1,   SUBSTR(C2,1,10) C2 FROM   T1 WHERE   C1=10000
 
Plan hash value: 1462557378
 
--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |       |       |     3 (100)|          |
|   1 |  TABLE ACCESS BY INDEX ROWID| T1           |     1 |   205 |     3   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | SYS_C0020654 |     1 |       |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("C1"=10000)
 
----
 
SQL_ID  b6g33508zt1nz, child number 0
-------------------------------------
SELECT   C1,   SUBSTR(C2,1,10) C2 FROM   T1 WHERE   NVL(C1,0)=10000
 
Plan hash value: 3617692013
 
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |       |       |  1507 (100)|          |
|*  1 |  TABLE ACCESS FULL| T1   |     1 |   205 |  1507   (5)| 00:00:08 |
--------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(NVL("C1",0)=10000)
 
----
 
SQL_ID  43fdysjbtscam, child number 0
-------------------------------------
SELECT /*+ INDEX(T1) */   C1,   SUBSTR(C2,1,10) C2 FROM   T1 WHERE   NVL(C1,0)=10000
 
Plan hash value: 4057798311
 
-------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |       |       |  1919 (100)|
|   1 |  TABLE ACCESS BY INDEX ROWID| T1           |     1 |   205 |  1919   (3)| 00:00:10
|*  2 |   INDEX FULL SCAN           | SYS_C0020654 |     1 |       |  1918   (3)| 00:00:10
-------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter(NVL("C1",0)=10000)

The unhinted execution plan from 11.1.0.7:

SQL_ID  b6g33508zt1nz, child number 0
-------------------------------------
SELECT   C1,   SUBSTR(C2,1,10) C2 FROM   T1 WHERE   NVL(C1,0)=10000

Plan hash value: 3617692013
 
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |       |       |  3543 (100)|          |
|*  1 |  TABLE ACCESS FULL| T1   |     1 |   205 |  3543   (2)| 00:00:15 |
--------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(NVL("C1",0)=10000)

The index hinted execution plan (note that the calculated cost is lower):

SQL_ID  43fdysjbtscam, child number 0
-------------------------------------
SELECT /*+ INDEX(T1) */   C1,   SUBSTR(C2,1,10) C2 FROM   T1 WHERE
NVL(C1,0)=10000

Plan hash value: 3968247418

--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |       |       |  1909 (100)|          |
|   1 |  TABLE ACCESS BY INDEX ROWID| T1           |     1 |   205 |  1909   (2)| 00:00:08 |
|*  2 |   INDEX FULL SCAN           | SYS_C0017730 |     1 |       |  1908   (2)| 00:00:08 |
--------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter(NVL("C1",0)=10000)

I will have to test 11.2.0.1/11.2.0.2 later to see if I am able to reproduce the behavior – thanks for the tip.

Best regards

Maxim