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')