Why CBO don't use function-base index when I use like and bind variable

Question

Hello
I have litle problem with function-base index and like with bind variable.
When I use like with bind variable, the CBO don't use my function-base index.
For example when I create table and index:
ALTER SESSION SET NLS_SORT='BINARY_CI';
ALTER SESSION SET NLS_COMP='LINGUISTIC';
alter session set nls_language='ENGLISH';
-- DROP TABLE TEST1;
CREATE TABLE TEST1 (K1 VARCHAR2(32));
create index test1_idx on test1(nlssort(K1,'nls_sort=BINARY_CI'));
INSERT INTO TEST1
SELECT OBJECT_NAME FROM ALL_OBJECTS;
COMMIT;
When I run:
ALTER SESSION SET NLS_SORT='BINARY_CI';
ALTER SESSION SET NLS_COMP='LINGUISTIC';
SELECT * FROM TEST1 WHERE K1 = 'abcd';
or
SELECT * FROM TEST1 WHERE K1 LIKE 'abcd%';
CBO use index.
PLAN_TABLE_OUTPUT
SQL_ID 4vrmp7cshbvqy, child number 1
SELECT * FROM TEST1 WHERE K1 LIKE 'abcd%'
Plan hash value: 1885706448
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST1 | 2 | 98 | 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | TEST1_IDX | 2 | | 1 (0)| 00:00:01 |
Predicate Information (identified by operation id):
2 - access("TEST1"."SYS_NC00002$">=HEXTORAW('6162636400') AND
"TEST1"."SYS_NC00002$"<HEXTORAW('6162636500') )
but when I run
SELECT * FROM TEST1 WHERE K1 LIKE :1;
CBO don't use index
PLAN_TABLE_OUTPUT
SQL_ID 9t461s1669gru, child number 0
SELECT * FROM TEST1 WHERE K1 LIKE :1
Plan hash value: 4122059633
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | | | 89 (100)| |
|* 1 | TABLE ACCESS FULL| TEST1 | 2 | 48 | 89 (3)| 00:00:02 |
Predicate Information (identified by operation id):
1 - filter("K1" LIKE :1)
What should I change to force CBO to use index.
I don't wont use index hint in query.
My oracle version:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE     11.2.0.1.0     Production
TNS for 64-bit Windows: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

Answer 1

OK. But why if I create normal index (create index test1_idx on test1(K1)) and return to default nls settings this same query use index.
PLAN_TABLE_OUTPUT
SQL_ID 9t461s1669gru, child number 0
SELECT * FROM TEST1 WHERE K1 LIKE :1
Plan hash value: 598212486
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | | | 1 (100)| |
|* 1 | INDEX RANGE SCAN| TEST1_IDX | 1 | 18 | 1 (0)| 00:00:01 |
Predicate Information (identified by operation id):
1 - access("K1" LIKE :1)
filter("K1" LIKE :1)
Note
- dynamic sampling used for this statement (level=2)
when index is function-base (create index test1_idx on test1(nlssort(K1,'nls_sort=BINARY_CI')))
PLAN_TABLE_OUTPUT
SQL_ID 9t461s1669gru, child number 1
SELECT * FROM TEST1 WHERE K1 LIKE :1
Plan hash value: 4122059633
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | | | 89 (100)| |
|* 1 | TABLE ACCESS FULL| TEST1 | 3 | 54 | 89 (3)| 00:00:02 |
Predicate Information (identified by operation id):
1 - filter("K1" LIKE :1)
Note
- dynamic sampling used for this statement (level=2)
when I create index with upper function "index test1_idx on test1(upper(K1))" the query use index
SELECT * FROM TEST1 WHERE upper(K1) LIKE :1
Plan hash value: 1885706448
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST1 | 4481 | 157K| 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | TEST1_IDX | 806 | | 1 (0)| 00:00:01 |
Predicate Information (identified by operation id):
2 - access("TEST1"."SYS_NC00002$" LIKE :1)
filter("TEST1"."SYS_NC00002$" LIKE :1)
Note
- dynamic sampling used for this statement (level=2)