如果一张表上有十几个索引，你有什么感受？显然会拖慢增、删、改的速度，不要指望开发人员能建好索引。我的处理方法是先监控很长的一段时间，看哪些索引没有用到，然后删除。但删除以后，如果发现某一天有用，那又要重新建，如果是大表，那就有些麻烦。现在11g提供一个新特性，不可见索引，可以建索引设置为不可见索引，CBO在评估执行计划的时候会忽略它，如果需要的时候，设置回来即可。
还有一种用途，你在调试一条SQL语句，要建一个索引测试，而你不想影响其他的会话，用不可见索引正是时候。
SQL> drop table test purge;
SQL> create table test as select * from dba_objects;SQL> create index ind_t_object_id on test(object_id);
SQL> exec dbms_stats.gather_table_stats(user,‘test‘,cascade => true);
SQL> set autotrace traceonly
SQL> select * from test where object_id = 10;
执行计划
----------------------------------------------------------
Plan hash value: 255872589
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 100 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 100 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=10)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> alter index ind_t_object_id invisible;
SQL> select * from test where object_id = 10;
执行计划
----------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 100 | 209 (1)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| TEST | 1 | 100 | 209 (1)| 00:00:03 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"=10)
统计信息
----------------------------------------------------------
196 recursive calls
0 db block gets
567 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select /*+ index(test ind_t_object_id)*/ * from test where object_id = 10;
执行计划
----------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 100 | 209 (1)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| TEST | 1 | 100 | 209 (1)| 00:00:03 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"=10)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
544 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--让数据库看到不可见索引，可以通过改变一个参数
SQL> alter session set optimizer_use_invisible_indexes = true;
SQL> select * from test where object_id = 10;
执行计划
----------------------------------------------------------
Plan hash value: 255872589
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 100 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 100 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=10)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed