固定执行计划--通过OUTLINE实现

卢光远

2023-12-01

SQL> select * from V$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

1. 正常的执行计划

SQL> create table t1 as select * from dba_objects;

SQL> set autot trace exp

1.1未创建索引的执行计划

SQL> select * from t1 where object_id=111;

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 37 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T1 | 1 | 177 | 37 (0)| 00:00:01 |
--------------------------------------------------------------------------

1.2创建了索引的执行计划

SQL> create index ind_t1 on t1(object_id);
SQL> select * from t1 where object_id=111;

--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 177 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T1 | 1 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------

2.使用了OUTLINE的执行计划

2.1 未指定category

SQL> drop index ind_t1;
SQL> select * from t1 where object_id=111;

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 37 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T1 | 1 | 177 | 37 (0)| 00:00:01 |
--------------------------------------------------------------------------

2.1.1未使用category创建outline

SQL> create outline o_t1_noc on
2 select * from t1 where object_id=111;

SQL> create index ind_t1 on t1(object_id);

SQL> select * from t1 where object_id=111;

--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 177 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T1 | 1 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------

可以看到索引生效,outline未生效。

2.1.2 指定category，让上面Outline生效

SQL> alter session set use_stored_outlines=default;
如果在创建outline时未指定category，则catagory为默认的default。也可以在这设为true以使用默认的default
本次使用alter session,全局指定用alter system

SQL> select * from t1 where object_id=111;

Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 107 | 18939 | 37 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T1 | 107 | 18939 | 37 (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("OBJECT_ID"=111)

Note
-----
- outline "O_T1_NOC" used for this statement

可以看到走了全表，并在最后提示使用o_t1_noc

2.2 使用有category的outline

SQL> drop index ind_t1;
SQL> drop outline o_t1_noc;

SQL> select * from t1 where object_id=111;

Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 37 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T1 | 1 | 177 | 37 (0)| 00:00:01 |
--------------------------------------------------------------------------

SQL> create index ind_t1 on t1(object_id);
SQL> select * from t1 where object_id=111;

Execution Plan
----------------------------------------------------------
Plan hash value: 634656657
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 177 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T1 | 1 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------

SQL> alter session set use_stored_outlines=c_t1;

SQL> select * from t1 where object_id=111;

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 107 | 18939 | 37 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T1 | 107 | 18939 | 37 (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("OBJECT_ID"=111)

Note
-----
- outline "O_T1_USEC" used for this statement

3.对outline的管理

3.1清空未用的Outline
exec DBMS_OUTLN.DROP_UNUSED
3.2 查看创建好的outline
select * from user_outlines;

3.2关于use_stored_outlines
To use stored outlines when Oracle compiles a SQL statement, set the system parameter USE_STORED_OUTLINES to true or to a category name. If you set USE_STORED_OUTLINES to true, then Oracle uses outlines in the default category. If you specify a category with the USE_STORED_OUTLINES parameter, then Oracle uses outlines in that category until you reset the parameter to another category name or until you suspend outline use by setting USE_STORED_OUTLINES to false

参考文章：

http://docs.oracle.com/cd/B19306_01/server.102/b14211/outlines.htm#sthref1341