关于Oracle降序索引的定意及回溯（转）

降序索引本质上是FBI ，其具体定义可以通过 user_ind_expressions 或dba_ind_expressions 查询 。
那么同样的 ，降序索引只有在CBO下才能被使用 。

Connected to Oracle9i Enterprise Edition Release 9.2.0.4.0
Connected as eygle


SQL> create table t as select * from dba_users;

Table created

SQL> create index idx_username_desc on t(username desc);

Index created


SQL> select index_name,table_name,INDEX_TYPE from user_indexes where table_name='T';

INDEX_NAME TABLE_NAME INDEX_TYPE
------------------------------ ------------------------------ ---------------------------
IDX_USERNAME_DESC T FUNCTION-BASED NORMAL


SQL> select column_name,column_position,descend from user_ind_columns
2 where table_name='T';

COLUMN_NAME COLUMN_POSITION DESCEND
------------------------------ --------------- -------
SYS_NC00013$ 1 DESC

SQL>


SQL> select * from user_ind_expressions where table_name='T';

INDEX_NAME TABLE_NAME COLUMN_EXPRESSION COLUMN_POSITION
------------------------------ ------------------------------ ------------------------------ ---------------
IDX_USERNAME_DESC T "USERNAME" 1

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