scott@TEST>create index idx_emp_mgr on emp(mgr);
Index created.
scott@TEST>create index idx_emp_dept on emp(deptno);
Index created.
scott@TEST>select /*+ index_combine(emp pk_emp idx_emp_mgr idx_emp_deptno) */ empno,ename,sal,job
2 from emp
3 where empno=7369 and mgr=7902 and deptno=20;
EMPNO ENAME SAL JOB
---------- ------------------------------ ---------- ---------------------------
7369 SMITH 800 CLERK
Execution Plan
----------------------------------------------------------
Plan hash value: 1816402415
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 29 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 29 | 2 (0)| 00:00:01 |
| 2 | BITMAP CONVERSION TO ROWIDS | | | | | |
| 3 | BITMAP AND | | | | | |
| 4 | BITMAP CONVERSION FROM ROWIDS| | | | | |
|* 5 | INDEX RANGE SCAN | PK_EMP | | | 0 (0)| 00:00:01 |
| 6 | BITMAP CONVERSION FROM ROWIDS| | | | | |
|* 7 | INDEX RANGE SCAN | IDX_EMP_MGR | | | 1 (0)| 00:00:01 |
| 8 | BITMAP CONVERSION FROM ROWIDS| | | | | |
|* 9 | INDEX RANGE SCAN | IDX_EMP_DEPT | | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
......
从上面的执行计划中可以看到关键字“BITMAP CONVERSION FROM ROWIDS”、“BITMAP AND”和“BITMAP CONVERSION TO ROWIDS”，这说明Oracle先分别对上述三个单键值的B*Tree索引IDX_EMP_MGR、IDX_EMP_DEPT和PK_EMP用映射函数将其中的ROWID转换成了位图，然后对转换后的位图执行了BITMAP AND(位图按位与)布尔运算，最后将布尔运算的结果再次用映射函数转换成了ROWID并回表得到最终的执行结果。能走出这样的执行计划显然是因为INDEX_COMBINE Hint生效了。
用映射函数将ROWID转换成了位图，然后再执行布尔运算，最后将布尔运算的结果再次用映射函数转换成了ROWID并回表得到最终的执行结果，这个过程在实际生产环境中的执行效率可能是有问题的，可以使用隐含参数_B_TREE_BITMAP_PLANS禁掉该过程中的ROWID到位图的转换：
alter session set "_b_tree_bitmap_plans"=false;
scott@TEST>alter session set "_b_tree_bitmap_plans"=false;
Session altered.
scott@TEST>select /*+ index_combine(emp pk_emp idx_emp_mgr idx_emp_deptno) */ empno,ename,sal,job
2 from emp
3 where empno=7369 and mgr=7902 and deptno=20;
EMPNO ENAME SAL JOB
---------- ------------------------------ ---------- ---------------------------
7369 SMITH 800 CLERK
Execution Plan
----------------------------------------------------------
Plan hash value: 2949544139
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 29 | 1 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 29 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
......
从上面的执行计划中可以看出没有出现BITMAP相关的关键字，即INDEX_COMBINE Hint被Oracle忽略了。
5、INDEX_FFS