Oracle Study---Oracle 11g 不可见索引案例


     Oracle 11g较之前的版本,推出了很多新功能,其中一项就是不可见索引(invisible index)。
    从Oracle 11g开始,可以创建不可见索引(invisible index),优化器会忽略不可见的索引。
初始化参数optimizer_use_invisible_indexes决定是否使用invisible index,其默认值为false,即默认不使用invisible index。但如果在session级别或者system级别上将optimizer_use_invisible_indexes初始化参数设置为true,那么就可以使用invisible index。
与不可用索引不同,不可见索引在使用DML语句期间仍会得到维护。
Oracle引入不可见索引是有用途的,使索引不可见是使索引不可用或者删除索引的一种替代办法。使用不可见的索引,可以完成如下操作:
1、在删除索引之前测试对索引的删除,是否会产生影响。
2、对应用程序的特定操作或模块是使用临时索引结构,这样就不会影响整个应用程序。
当索引不可见时,优化器生成的执行计划不会使用该索引。删除索引时,可以先将索引修改为invisible,如果未发生性能下降的问题,则可以删除该索引。在表上新建索引时,可以先创建一个最初不可见的索引,然后执行测试,看索引的效率怎么样,最后确定是否使该索引可见,是否使用该索引。
可以查看dba_indexes、all_indexes、user_indexes视图的visibility字段来确定该索引是visible还是invisible。

1、创建索引
11:47:59 SCOTT@ enmo> create index emp1_name_ind on emp1(ename) tablespace indx;
Index created.
Elapsed: 00:00:00.07
11:50:13 SCOTT@ enmo> exec dbms_stats.gather_table_stats(user,'emp1',cascade=>true);
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.08
2、设置索引不可见
11:53:47 SCOTT@ enmo> ALTER INDEX emp1_name_ind invisible;
Index altered.
Elapsed: 00:00:00.05
3、在执行查询时,优化器没有选择索引
11:56:17 SCOTT@ enmo> SELECT * FROM EMP1 WHERE ENAME='SCOTT';
     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7788 SCOTT      ANALYST         7566 19-APR-87       3000                    20
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 2226897347
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    38 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP1 |     1 |    38 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("ENAME"='SCOTT')


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
        863  bytes sent via SQL*Net to client
        415  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
4、设置优化参数
11:58:33 SYS@ enmo> grant alter session to scott;

Grant succeeded.


11:56:34 SCOTT@ enmo> alter session set optimizer_use_invisible_indexes=true;
Session altered.

Elapsed: 00:00:00.00


11:59:45 SCOTT@ enmo> SELECT * FROM EMP1 WHERE ENAME='SCOTT';
     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7788 SCOTT      ANALYST         7566 19-APR-87       3000                    20
Elapsed: 00:00:00.00

Execution Plan
----------------------------------------------------------
Plan hash value: 1963203073
---------------------------------------------------------------------------------------------
| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |               |     1 |    38 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP1          |     1 |    38 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EMP1_NAME_IND |     1 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ENAME"='SCOTT')


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        867  bytes sent via SQL*Net to client
        415  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
不可见索引被优化器启用
5、将索引变为可见,则优化器使用索引访问
11:59:56 SCOTT@ enmo> alter index emp1_name_ind visible;
Index altered.

Elapsed: 00:00:00.06


12:00:46 SCOTT@ enmo> alter session set optimizer_use_invisible_indexes=false;
Session altered.

Elapsed: 00:00:00.00


12:00:58 SCOTT@ enmo> select * from emp1 where ename='SCOTT';

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7788 SCOTT      ANALYST         7566 19-APR-87       3000                    20
Elapsed: 00:00:00.01


Execution Plan
----------------------------------------------------------
Plan hash value: 1963203073
---------------------------------------------------------------------------------------------
| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |               |     1 |    38 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP1          |     1 |    38 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EMP1_NAME_IND |     1 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("ENAME"='SCOTT')

Statistics
----------------------------------------------------------
        189  recursive calls
          0  db block gets
         26  consistent gets
          0  physical reads
          0  redo size
        867  bytes sent via SQL*Net to client
        415  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          6  sorts (memory)
          0  sorts (disk)
          1  rows processed
6、使用hints,不可见索引也不能被启用 

12:01:10 SCOTT@ enmo> ALTER INDEX emp1_name_ind invisible;
Index altered.

12:02:49 SCOTT@ enmo> select table_name,index_name,leaf_blocks,status,VISIBILITY FROM USER_INDEXES
12:06:03   2  where table_name='EMP1';
TABLE_NAME                     INDEX_NAME                     LEAF_BLOCKS STATUS   VISIBILIT
------------------------------ ------------------------------ ----------- -------- ---------
EMP1                           EMP1_NAME_IND                            1 VALID    INVISIBLE

12:26:11 SCOTT@ enmo>  select /*+ index(emp1 emp1_name_ind) */ * from emp1 where ename='SCOTT'
     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7788 SCOTT      ANALYST         7566 19-APR-87       3000                    20
Elapsed: 00:00:00.01
Execution Plan
----------------------------------------------------------
Plan hash value: 2226897347
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    38 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP1 |     1 |    38 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("ENAME"='SCOTT')

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
        863  bytes sent via SQL*Net to client
        415  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed