规则如下：

     依据这个规则、只有 3 种输出：大写、小写、首字母大写
     
     测试如下：

sys@ORCL> select to_char(sysdate,\'Month\') from dual;

TO_CHAR(SYSDATE,\'MONTH\')
------------------------------------
June

sys@ORCL> select to_char(sysdate,\'MOnth\') from dual;

TO_CHAR(SYSDATE,\'MONTH\')
------------------------------------
JUNE

sys@ORCL> select to_char(sysdate,\'month\') from dual;

TO_CHAR(SYSDATE,\'MONTH\')
------------------------------------
june

     下面做个测试、确认这个规则对函数索引的影响：

sys@ORCL> drop table t purge;

Table dropped.

sys@ORCL> create table t as select object_id,sysdate+rownum as create_date from dba_objects where rownum<=5000;

Table created.

sys@ORCL> analyze table t compute statistics;

Table analyzed.

sys@ORCL> set autot trace exp
sys@ORCL> select * from t where to_char(create_date,\'yyyy-mm-dd\')=\'2011-06-02\';

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    50 |   500 |     5  (20)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |    50 |   500 |     5  (20)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter(TO_CHAR(INTERNAL_FUNCTION("CREATE_DATE"),\'yyyy-mm-dd\')=\'20
              11-06-02\')

sys@ORCL> create index idx_t on t (to_char(create_date,\'YYYY-MM-DD\'));

Index created.

sys@ORCL> analyze index idx_t validate structure;

Index analyzed.

sys@ORCL> select * from t where to_char(create_date,\'yyyy-mm-dd\')=\'2013-06-02\';

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    50 |   500 |     5  (20)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |    50 |   500 |     5  (20)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter(TO_CHAR(INTERNAL_FUNCTION("CREATE_DATE"),\'yyyy-mm-dd\')=\'20
              13-06-02\')

sys@ORCL> set autot off
sys@ORCL> create index idx_t_001 on t (to_char(create_date,\'yyyy-mm-dd\'));

Index created.

sys@ORCL> analyze index idx_t_001 validate structure;

Index analyzed.

sys@ORCL> select * from t where to_char(create_date,\'yyyy-mm-dd\')=\'2013-06-02\';

no rows selected

sys@ORCL> set autot trace exp

第一种情况：小写

sys@ORCL> select * from t where to_char(create_date,\'yyyy-mm-dd\')=\'2013-06-02\';

Execution Plan
----------------------------------------------------------
Plan hash value: 512271049

-----------------------------------------------------------------------------------------
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |           |    50 |   500 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T         |    50 |   500 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX_T_001 |    20 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   2 - access(TO_CHAR(INTERNAL_FUNCTION("CREATE_DATE"),\'yyyy-mm-dd\')=\'2013-06-02\'
              )


第二种情况：首字母大写

sys@ORCL> select * from t where to_char(create_date,\'yyyy-mm-Dd\')=\'2013-06-02\';

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    50 |   500 |     5  (20)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |    50 |   500 |     5  (20)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter(TO_CHAR(INTERNAL_FUNCTION("CREATE_DATE"),\'yyyy-mm-Dd\')=\'20
              13-06-02\')


第三种情况：大写

sys@ORCL> select * from t where to_char(create_date,\'yyyy-mm-DD\')=\'2013-06-02\';

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    50 |   500 |     5  (20)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |    50 |   500 |     5  (20)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter(TO_CHAR(INTERNAL_FUNCTION("CREATE_DATE"),\'yyyy-mm-DD\')=\'20
              13-06-02\')