Oracle 11g新特性invisible index(不可见的索引)[通俗易懂]

    假设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。不要指望开发者能建好索引。我的处理方法是先监控非常长的一段时间。看哪些索引没实用到，然后删除。

但删除以后，假设发现某一天实用，那又要又一次建，假设是大表。那就有些麻烦。如今11g提供一个新特性，不可见索引。能够建索引设置为不可见索引。CBO在评估运行计划的时候会忽略它，假设须要的时候。设置回来就可以。

    另一种用途，你在调试一条SQL语句，要建一个索引測试。而你不想影响其它的会话，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;
运行计划
———————————————————-
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;
运行计划
———————————————————-
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;
运行计划
———————————————————-
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，能够通过改变一个參数
SQL> alter session set optimizer_use_invisible_indexes = true;
SQL> select * from test where object_id = 10;
运行计划
———————————————————-
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