oracle性能調優-虛擬索引

一、引言

    DBA在日常維護管理數據庫進行低性能SQL分析時，有時候需要通過創建索引對SQL進行優化，但有些時候我們創建的索引是否能用到？這個只能創建以后才能看出效果，但是在實際工作中，特別是對大表創建索引對系統性能有很大影響，因此我們不得不避開業務高峰時段，但是有沒有一種辦法創建索引而不影響性能呢？有，那就是虛擬索引。

    虛擬索引不是物理存在的，它并不會創建實際的索引段，只是在數據字典中加了一個索引的記錄，使得優化器能夠意識到一個索引的存在，從而判斷是否使用該索引作為訪問路徑。作用僅僅是為了DBA作SQL優化時使用，DBA根據虛擬索引的優化效果決定是否創建物理索引。

二、虛擬索引類型

    虛擬索引支持B-TREE索引和BIT位圖索引，在CBO模式下ORACLE優化器會考慮虛擬索引，但是在RBO模式下需要添加hint才行。

三、虛擬索引創建實例

SQL> create table scott.t as select * from dba_objects;

Table created.

SQL> alter session set "_use_nosegment_indexes"=true;

Session altered.

SQL> create index scott.ix_t_id on scott.t(object_id) nosegment;

Index created.

SQL> set autot traceonly

SQL> select * from scott.t where object_id=1;

no rows selected

Execution Plan

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

Plan hash value: 206018885

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

| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT            |         |    14 |  2898 |     5   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T       |    14 |  2898 |     5   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IX_T_ID |   330 |       |     1   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

   2 - access("OBJECT_ID"=1)

Note

-----

   - dynamic sampling used for this statement (level=2)

Statistics

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

          4  recursive calls

          0  db block gets

       1308  consistent gets

       1239  physical reads

          0  redo size

       1343  bytes sent via SQL*Net to client

        512  bytes received via SQL*Net from client

          1  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          0  rows processed

-- 以下看的是真實執行計劃，顯然是用不到索引。

SQL> set autot off

SQL> alter session set statistics_level=all;

Session altered.

SQL> select * from scott.t where object_id=1;

no rows selected

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT

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

SQL_ID  2qhwh0nzrzx2r, child number 1

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

select * from t where object_id=1

Plan hash value: 1601196873

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

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |

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

|   0 | SELECT STATEMENT  |      |      1 |        |      0 |00:00:00.52 |    1242 |   1239 |

|*  1 |  TABLE ACCESS FULL| T    |      1 |     14 |      0 |00:00:00.52 |    1242 |   1239 |

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

Predicate Information (identified by operation id):

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

   1 - filter("OBJECT_ID"=1)

Note

-----

   - dynamic sampling used for this statement (level=2)

22 rows selected.

-- 從數據字段中是無法找到這個索引的。

SQL> select index_name,status from dba_indexes where table_name='T';

no rows selected

四、虛擬索引的特點

    4.1、虛擬索引無法執行alter index選項

SQL> alter index scott.IX_T_ID rebuild;

alter index scott.IX_T_ID rebuild

*

ERROR at line 1:

ORA-08114: can not alter a fake index

    4.2、使用回收站特性的時候，虛擬索引必須先drop，才能創建同名的索引。

SQL> drop table scott.t;

Table dropped.

SQL> flashback table scott.t to before drop;

Flashback complete.

SQL> create index scott.idx_t_id on scott.t(object_id) nosegment;

create index scott.idx_t_id on scott.t(object_id) nosegment

                  *

ERROR at line 1:

ORA-00955: name is already used by an existing object

    4.3、不能創建和虛擬索引同名的實際索引；

    4.4、可以創建和虛擬索引包含相同列但不同名的實際索引;

    4.5、虛擬索引在數據字典里看不到