http://www.itpub.net/thread-1350771-1-1.html

在上面的帖子里，楼主提出使用dbms_session来处理绑定变量。但是，这个方法必须进行有效的测试，防止出现错误的执行计划。

由于sys_context导致bind_peeking无法正确执行，在某些情况下，会生成错误的执行计划，见下例：

[@more@]

SQL*Plus: Release 11.2.0.1.0 Production on 星期三 9月 29 08:43:56 2010

Copyright (c) 1982, 2010, Oracle. All rights reserved.

SUNDOG315>conn test/test
已连接。
SUNDOG315>create table t (id number,text varchar2(4000));

表已创建。

SUNDOG315>insert into t select 1,object_name from dba_objects;

已创建55085行。

SUNDOG315>insert into t select 2,'aaa' from dual;

已创建 1 行。

SUNDOG315>commit;

提交完成。

SUNDOG315>create index t_idx on t(id);

索引已创建。

SUNDOG315>exec dbms_stats.gather_table_stats(user,'T',null,null,method_opt=>'for
all columns',cascade=>true);

PL/SQL 过程已成功完成。

SUNDOG315>select id,count(*) from t group by id;

ID COUNT(*)
---------- ----------
1 55085
2 1

SUNDOG315>select count(*) from t where id=2;

COUNT(*)
----------
1

SUNDOG315>select * from table(dbms_xplan.display_cursor());

SQL_ID 8tfp2xac71yd6, child number 0
-------------------------------------
select count(*) from t where id=2

Plan hash value: 293504097

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| T_IDX | 1 | 3 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------

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

2 - access("ID"=2)

已选择19行。

--选择了正确的索引范围扫描

SUNDOG315>var fid number;
SUNDOG315>exec :fid := 2;

PL/SQL 过程已成功完成。


SUNDOG315>select count(*) from t where id=:fid;

COUNT(*)
----------
1

SUNDOG315>select * from table(dbms_xplan.display_cursor());

SQL_ID 3u1847dua6k75, child number 0
-------------------------------------
select count(*) from t where id=:fid

Plan hash value: 293504097

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| T_IDX | 1 | 3 | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------

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

2 - access("ID"=:FID)

已选择19行。

--由于bind_peeking的存在，依然选择了正确的索引范围扫描

SUNDOG315>create context test using test_p;

上下文已创建。

SUNDOG315>create or replace procedure test_p is
2 begin
3 dbms_session.set_context('test','id',2);
4 end;
5 /

过程已创建。

SUNDOG315>exec test_p;

PL/SQL 过程已成功完成。

SUNDOG315>select sys_context('TEST','ID') from dual;

SYS_CONTEXT('TEST','ID')
--------------------------------------------------------------------------------

2

SUNDOG315>select count(*) from t where id=sys_context('test','id');

COUNT(*)
----------
1

SUNDOG315>select * from table(dbms_xplan.display_cursor());

SQL_ID 69x691fhtckm0, child number 0
-------------------------------------
select count(*) from t where id=sys_context('test','id')

Plan hash value: 1058879072

-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 32 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX FAST FULL SCAN| T_IDX | 27543 | 82629 | 32 (4)| 00:00:01 |
-------------------------------------------------------------------------------

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

2 - filter("ID"=TO_NUMBER(SYS_CONTEXT('test','id')))

已选择19行。

--TO_NUMBER(SYS_CONTEXT('test','id'))无法peeking到正确的值，因此，选择了错误的执行计划

SUNDOG315>set autot trace stat
SUNDOG315>select count(*) from t where id=2;


统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
423 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

SUNDOG315>select count(*) from t where id=:fid;


统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
423 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

SUNDOG315>select count(*) from t where id=sys_context('test','id');


统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
116 consistent gets --逻辑读增大
0 physical reads
0 redo size
423 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

目前，估计用dbms_session的人还是比较少，像这种问题，Oracle其实完全可以处理掉的。估计如果使用的人多，就会进行处理了。呵呵