在Oracle 12c中，Oracle提供了可以在同一个表的列上建立不同的索引，用来做SQL的性能优化，但是仅有一个索引是可见的。
下面是一个示例
Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options
SQL> alter session set container=pdb1;
Session altered.
SQL> CREATE TABLE t (
2 id NUMBER,
3 ename VARCHAR2(50)
4 );
Table created.
SQL> CREATE INDEX t_idx1 ON t(ename) VISIBLE;
Index created.
SQL> CREATE INDEX t_idx2 ON t(ename) INVISIBLE;
CREATE INDEX t_idx2 ON t(ename) INVISIBLE
*
ERROR at line 1:
ORA-01408: such column list already indexed
SQL> CREATE BITMAP INDEX t_idx3 ON t(ename) INVISIBLE;
Index created.
SQL> l
1 SELECT a.index_name,
2 a.index_type,
3 a.partitioned,
4 b.partitioning_type,
5 b.locality,
6 a.visibility
7 FROM user_indexes a
8 LEFT OUTER JOIN user_part_indexes b ON a.index_name = b.index_name where a.table_name='T'
9* ORDER BY index_name
SQL> /
INDEX_NAME INDEX_TYPE PARTITIONED PARTITIONING_TYPE LOCALITY VISIBILITY
---------- ---------- ------------ --------------------------- -------- ----------
T_IDX1 NORMAL NO VISIBLE
T_IDX3 BITMAP NO INVISIBLE