从ORACLE 12C开始允许在同一列上创建不同类型的index，用来做性能调优，不同类型的index主要包括（B*TREE vs BITMAP，Local vs Global)
ORACLE 11G

SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - Production
PL/SQL Release 11.2.0.3.0 - Production
CORE    11.2.0.3.0      Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
SQL> CREATE TABLE T_XIFENFEI
  2    (
  3    EMPNO      NUMBER(9)       PRIMARY KEY ,
  4    EMPNAME    VARCHAR2(15)    NOT NULL,
  5    )
  6    /
Table created.
SQL> CREATE INDEX EMP_DEPT_BTREE_IDX ON EMP(DEPTNO);
Index created.
SQL> CREATE BITMAP INDEX EMP_DEPTNO_BITMAP_IDX ON EMP(DEPTNO) INVISIBLE  ;
CREATE BITMAP INDEX EMP_DEPTNO_BITMAP_IDX ON EMP(DEPTNO) INVISIBLE
                                                 *
ERROR at line 1:
ORA-01408: such column list already indexed

ORACLE 12C

CDB_PDB@CHF> select * from v$version;
BANNER                                                                               CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production              0
PL/SQL Release 12.1.0.1.0 - Production                                                    0
CORE    12.1.0.1.0      Production                                                        0
TNS for 64-bit Windows: Version 12.1.0.1.0 - Production                                   0
NLSRTL Version 12.1.0.1.0 - Production                                                    0
CDB_PDB@CHF> CREATE TABLE T_XIFENFEI
  2    (
  3    EMPNO      NUMBER(9)       PRIMARY KEY ,
  4    EMPNAME    VARCHAR2(15)    NOT NULL
  5    )
  6    /
表已创建。
CDB_PDB@CHF> CREATE INDEX EMP_DEPT_BTREE_IDX ON EMP(DEPTNO);
索引已创建。
CDB_PDB@CHF> CREATE BITMAP INDEX EMP_DEPTNO_BITMAP_IDX ON EMP(DEPTNO);
CREATE BITMAP INDEX EMP_DEPTNO_BITMAP_IDX ON EMP(DEPTNO)
                                                 *
第 1 行出现错误:
ORA-01408: 此列列表已索引
CDB_PDB@CHF> CREATE BITMAP INDEX EMP_DEPTNO_BITMAP_IDX ON EMP(DEPTNO) INVISIBLE ;
索引已创建。