深入理解Oracle索引(15)：日期转换函数的格式参数大小写规则对函数索引的影响

     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')------------------------------------Junesys@ORCL> select to_char(sysdate,'MOnth') from dual;TO_CHAR(SYSDATE,'MONTH')------------------------------------JUNEsys@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 expsys@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 offsys@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 selectedsys@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')