Creating an Invisible Index

OCP:053 

22.An index called ORD_CUSTNAME_IX has been created on the CUSTNAME column in the ORDERS

table using the following command:

SQL>CREATE INDEX ord_custname_ix ON orders(custname);

The ORDERS table is frequently queried using the CUSTNAME column in the WHERE clause. You want

to check the impact on the performance of the queries if the index is not available. You do not want the

index to be dropped or rebuilt to perform this test.

Which is the most efficient method of performing this task?

A. disabling the index

B. making the index invisible

C. making the index unusable

D. using the MONITORING USAGE clause for the index

Answer: B

Creating an Invisible Index

An invisible index is an index that is ignored by the optimizer unless you explicitly set the OPTIMIZER_USE_INVISIBLE_INDEXES initialization parameter to TRUE at the session or system level.

To create an invisible index: 

• Use the CREATE INDEX statement with the INVISIBLE keyword.

  The following statement creates an invisible index named emp_ename for the ename column of the emp table:

  CREATE INDEX emp_ename ON emp(ename)

        TABLESPACE users

        STORAGE (INITIAL 20K

        NEXT 20k) INVISIBLE;

 

隐藏索引

scott@TESTDB> create index emp_ename_i on emp(ename) invisible;

 

Index created.

 

 

scott@TESTDB> select index_name,VISIBILITY from user_indexes; 

 

INDEX_NAME           VISIBILIT

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

PK_EMP               VISIBLE

EMP_SAL_F            VISIBLE

EMP_COMM_I           VISIBLE

EMP_ENAME_I          INVISIBLE

PK_DEPT              VISIBLE

 

scott@TESTDB> select * from emp where ename='KING';

 

 

没有走索引

 

切换到系统用户，修改参数

sys@TESTDB> alter session set optimizer_use_invisible_indexes=true;

 

Session altered.

sys@TESTDB> select * from scott.emp where ename='KING';

 

 

 

隐藏索引变正常索引或反之

sys@TESTDB> alter index scott.emp_ename_i visible;

 

Index altered.

 

scott@TESTDB>  select index_name,VISIBILITY from user_indexes;

 

INDEX_NAME                     VISIBILIT

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

PK_EMP                         VISIBLE

EMP_SAL_F                      VISIBLE

EMP_COMM_I                     VISIBLE

EMP_ENAME_I                    VISIBLE

PK_DEPT                        VISIBLE

 

 

多个索引，把慢的索引隐藏点，让他走快的索引

 

 

scott@TESTDB> alter index emp_ename_i visible;

 

Index altered.

 

scott@TESTDB> alter index emp_ename_i invisible;

 

Index altered.