11g新特性_索引的可見與不可見

當你想評估一個索引對你SQL查詢語句的影響時，恰巧你的數據庫是11g時候，可以快速將索引設置成VISIBLE或INVISIBLE。值得稱讚的時，當索引被設置成INVISIBLE時候，DML語句發生時候照常會維護索引，也就是說處於INVISIBLE狀態下的索引並不失效，只是優化器不選擇索引路徑而已。

 

下面通過實驗來驗證一下：

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
PL/SQL Release 11.2.0.4.0 - Production
CORE	11.2.0.4.0	Production
TNS for Linux: Version 11.2.0.4.0 - Production
NLSRTL Version 11.2.0.4.0 - Production

SQL> select * from t;

	ID
----------
	 2
	11
	22
	 1
	33
SQL> select count(id) from t;

 COUNT(ID)
----------
	 5


Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522

---------------------------------------------------------------------------
| Id  | Operation	   | Name | Rows  | Bytes | Cost (%CPU)| Time	  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	  |	1 |	3 |	2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |	  |	1 |	3 |	       |	  |
|   2 |   TABLE ACCESS FULL|  T	  |	5 |    15 |	2   (0)| 00:00:01 |  --可以看到此時是全表掃描

 

在T表ID列創建唯一索引

 

<p>SQL> create unique index idx_t_id on t(id) invisible;</p><p>Index created.</p>SQL> select count(id) from t;      

 COUNT(ID)
----------
	 5


Execution Plan
----------------------------------------------------------
Plan hash value: 4168287108

-----------------------------------------------------------------------------
| Id  | Operation	 | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |	    |	  1 |	  3 |	  1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |	    |	  1 |	  3 |		 |	    |
|   2 |   INDEX FULL SCAN| IDX_T_ID |	  5 |	 15 |	  1   (0)| 00:00:01 |
-----------------------------------------------------------------------------  --發生索引全掃描

將索引設置成INVISIBLE

SQL> alter index idx_t_id invisible;
 
Index altered<p>SQL> select index_name,status,visibility from dba_indexes where table_name='T';
 
INDEX_NAME                     STATUS   VISIBILITY
------------------------------ -------- ----------
IDX_T_ID                       VALID     INVISIBLE<p>SQL> select count(id) from t;
COUNT(ID)
----------
  5</p><p> </p>Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522</p><p>---------------------------------------------------------------------------
| Id  | Operation    | Name | Rows  | Bytes | Cost (%CPU)| Time   |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |   | 1 | 3 | 2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |   | 1 | 3 |        |   |
|   2 |   TABLE ACCESS FULL| T | 5 |15 | 2   (0)| 00:00:01 |      --此時執行計劃裏又是走全表掃描路徑
--------------------------------------------------------------------------- 
SQL> alter index idx_t_id visible;
Index altered

索引的不可見可以避免了索引重建，尤其是大表的索引，這個新特性更有利於數據庫的優化

 

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