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 | TABLE ACCESS FULL| 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 | INDEX FULL SCAN| 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 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 | --此时执行计划里又是走全表扫描路径
---------------------------------------------------------------------------
SQL> alter index idx_t_id visible;
Index altered
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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