Oracle Study---Oracle 11g 不可见索引案例

时间 2020-01-23

原文原文链接

Oracle Study---Oracle 11g 不可见索引案例session

Oracle 11g较以前的版本，推出了不少新功能，其中一项就是不可见索引（invisible index）。
从Oracle 11g开始，能够建立不可见索引（invisible index），优化器会忽略不可见的索引。
初始化参数optimizer_use_invisible_indexes决定是否使用invisible index，其默认值为false，即默认不使用invisible index。但若是在session级别或者system级别上将optimizer_use_invisible_indexes初始化参数设置为true，那么就可使用invisible index。
与不可用索引不一样，不可见索引在使用DML语句期间仍会获得维护。
Oracle引入不可见索引是有用途的，使索引不可见是使索引不可用或者删除索引的一种替×××法。使用不可见的索引，能够完成以下操做：
一、在删除索引以前测试对索引的删除，是否会产生影响。
二、对应用程序的特定操做或模块是使用临时索引结构，这样就不会影响整个应用程序。
当索引不可见时，优化器生成的执行计划不会使用该索引。删除索引时，能够先将索引修改成invisible，若是未发生性能降低的问题，则能够删除该索引。在表上新建索引时，能够先建立一个最初不可见的索引，而后执行测试，看索引的效率怎么样，最后肯定是否使该索引可见，是否使用该索引。
能够查看dba_indexes、all_indexes、user_indexes视图的visibility字段来肯定该索引是visible仍是invisible。

一、建立索引
11:47:59 SCOTT@ enmo> create index emp1_name_ind on emp1(ename) tablespace indx;
Index created.
Elapsed: 00:00:00.07
11:50:13 SCOTT@ enmo> exec dbms_stats.gather_table_stats(user,'emp1',cascade=>true);
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.08
二、设置索引不可见
11:53:47 SCOTT@ enmo> ALTER INDEX emp1_name_ind invisible;
Index altered.
Elapsed: 00:00:00.05
三、在执行查询时，优化器没有选择索引
11:56:17 SCOTT@ enmo> SELECT * FROM EMP1 WHERE ENAME='SCOTT';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 2226897347
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 38 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| EMP1 | 1 | 38 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
ide

1 - filter("ENAME"='SCOTT')性能

Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
863 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
四、设置优化参数
11:58:33 SYS@ enmo> grant alter session to scott;
测试

Grant succeeded.优化

11:56:34 SCOTT@ enmo> alter session set optimizer_use_invisible_indexes=true;
Session altered.
spa

Elapsed: 00:00:00.00orm

11:59:45 SCOTT@ enmo> SELECT * FROM EMP1 WHERE ENAME='SCOTT';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
Elapsed: 00:00:00.00

Execution Plan
----------------------------------------------------------
Plan hash value: 1963203073
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP1 | 1 | 38 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | EMP1_NAME_IND | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
索引

2 - access("ENAME"='SCOTT')ip

Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
867 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
不可见索引被优化器启用
五、将索引变为可见，则优化器使用索引访问
11:59:56 SCOTT@ enmo> alter index emp1_name_ind visible;
Index altered.
get

Elapsed: 00:00:00.06

12:00:46 SCOTT@ enmo> alter session set optimizer_use_invisible_indexes=false;
Session altered.

Elapsed: 00:00:00.00

12:00:58 SCOTT@ enmo> select * from emp1 where ename='SCOTT';

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
Elapsed: 00:00:00.01

Execution Plan
----------------------------------------------------------
Plan hash value: 1963203073
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP1 | 1 | 38 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | EMP1_NAME_IND | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ENAME"='SCOTT')

Statistics
----------------------------------------------------------
189 recursive calls
0 db block gets
26 consistent gets
0 physical reads
0 redo size
867 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
1 rows processed六、使用hints，不可见索引也不能被启用 12:01:10 SCOTT@ enmo> ALTER INDEX emp1_name_ind invisible;Index altered.12:02:49 SCOTT@ enmo> select table_name,index_name,leaf_blocks,status,VISIBILITY FROM USER_INDEXES12:06:03 2 where table_name='EMP1';TABLE_NAME INDEX_NAME LEAF_BLOCKS STATUS VISIBILIT------------------------------ ------------------------------ ----------- -------- ---------EMP1 EMP1_NAME_IND 1 VALID INVISIBLE12:26:11 SCOTT@ enmo> select /*+ index(emp1 emp1_name_ind) */ * from emp1 where ename='SCOTT' EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO---------- ---------- --------- ---------- --------- ---------- ---------- ---------- 7788 SCOTT ANALYST 7566 19-APR-87 3000 20Elapsed: 00:00:00.01Execution Plan----------------------------------------------------------Plan hash value: 2226897347--------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 1 | 38 | 3 (0)| 00:00:01 ||* 1 | TABLE ACCESS FULL| EMP1 | 1 | 38 | 3 (0)| 00:00:01 |--------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 1 - filter("ENAME"='SCOTT')Statistics---------------------------------------------------------- 0 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 863 bytes sent via SQL*Net to client 415 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed