这里, ORACLE只用到了DEPT_NO索引. 执行路径如下:

     TABLE ACCESS BY ROWID ON EMP

          INDEX RANGE SCAN ON DEPT_IDX

译者按:

我们来试一下以下这种情况:

SQL> select index_name, uniqueness from user_indexes where table_name = ''''EMP'''';

INDEX_NAME                     UNIQUENES

------------------------------ ---------

EMPNO                          UNIQUE

EMPTYPE                        NONUNIQUE

SQL> select * from emp where empno >= 2 and emp_type = ''''A'''' ;

no rows selected

Execution Plan

----------------------------------------------------------

   0      SELECT STATEMENT Optimizer=CHOOSE

   1    0   TABLE ACCESS (BY INDEX ROWID) OF ''''EMP''''

   2    1     INDEX (RANGE SCAN) OF ''''EMPTYPE'''' (NON-UNIQUE)

虽然EMPNO是唯一性索引,但是由于它所做的是范围比较, 等级要比非唯一性索引的等式比较低!

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