11g新特性_索引的可见与不可见

当你想评估一个索引对你SQL查询语句的影响时，恰巧你的数据库是11g时候，可以快速将索引设置成VISIBLE或INVISIBLE。值得称赞的时，当索引被设置成INVISIBLE时候，DML语句发生时候照常会维护索引，也就是说处于INVISIBLE状态下的索引并不失效，只是优化器不选择索引路径而已。

下面通过实验来验证一下：

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 |   <strong>TABLE ACCESS FULL</strong>| T	  |	5 |    15 |	2   (0)| 00:00:01 |--可以看到此时是全表扫描

在T表ID列创建唯一索引

<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 |   <strong>INDEX FULL SCAN</strong>| IDX_T_ID |	  5 |	 15 |	  1   (0)| 00:00:01 |
-----------------------------------------------------------------------------  --发生索引全扫描

将索引设置成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                       <span style="color:#ff0000;"><strong>VALID</strong></span>    <strong>INVISIBLE</strong></p><p>SQL> select count(id) from t;</p><p> COUNT(ID)
----------
  5</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 |   <strong>TABLE ACCESS FULL</strong>| T   | 5 |    15 | 2   (0)| 00:00:01 |      --此时执行计划里又是走全表扫描路径
---------------------------------------------------------------------------</p><p>SQL> alter index idx_t_id visible;
 
Index altered</p>

索引的不可见可以避免了索引重建，尤其是大表的索引，这个新特性更有利于数据库的优化

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