[20160201]db_link與子光標問題.txt

--生產系統遇到一個關于db_link產生大量子光標問題，當cursor_sharing=force的情況下，通過測試說明。
--注：這個問題我的測試僅僅存在10.2.0.4,11.2.0.4沒有這個問題。

1.環境：
SCOTT@test> @&r/ver1

PORT_STRING??????????????????? VERSION??????? BANNER
------------------------------ -------------- ----------------------------------------------------------------
x86_64/Linux 2.4.xx??????????? 10.2.0.4.0???? Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi

SCOTT@test> CREATE PUBLIC DATABASE LINK loopback USING '192.168.100.33:1521/test';
Database link created.

create or replace view v_emp as select * from emp@loopback;
create or replace view vv_emp as select * from v_emp;
create or replace view vvv_emp as select * from emp@loopback;

2.測試:
SCOTT@test> alter system flush shared_pool;
System altered.

SCOTT@test> alter session set cursor_sharing=force ;
Session altered.

--執行如下語句:
select * from emp@loopback where empno=1;
select * from emp@loopback where empno=2;
select * from emp@loopback where empno=3;

SCOTT@test> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
SQL_ID? 4zzhmns2wsf4g, child number 2
select * from emp@loopback where empno=:"SYS_B_0"
NOTE: cannot fetch plan for SQL_ID: 4zzhmns2wsf4g, CHILD_NUMBER: 2
????? Please verify value of SQL_ID and CHILD_NUMBER;
????? It could also be that the plan is no longer in cursor cache (check v$sql_plan)
8 rows selected.

SCOTT@test> select count(*) from v$sql where sql_id='4zzhmns2wsf4g';
? COUNT(*)
----------
???????? 3

--當sql語句僅僅含有遠程表，無法通過dbms_xplan.display_cursor獲得執行計劃。
--可以發現產生了3個子光標。當大量相似的sql語句執行時(在cursor_sharing=force的情況下)會產生大量的子光標。

--換成如下語句依舊：
select * from v_emp where empno=1;
select * from v_emp where empno=2;
select * from v_emp where empno=3;

SCOTT@test> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
SQL_ID? ftmcqat5qjmzc, child number 2

select * from v_emp where empno=:"SYS_B_0"

NOTE: cannot fetch plan for SQL_ID: ftmcqat5qjmzc, CHILD_NUMBER: 2
????? Please verify value of SQL_ID and CHILD_NUMBER;
????? It could also be that the plan is no longer in cursor cache (check v$sql_plan)

SCOTT@test> select count(*) from v$sql where sql_id='ftmcqat5qjmzc';
? COUNT(*)
----------
???????? 3

3.如果改用綁定變量看看：

variable x number ;
exec :x :=1;
select * from v_emp where empno=:x;
exec :x :=2;
select * from v_emp where empno=:x;
exec :x :=3;
select * from v_emp where empno=:x;

SCOTT@test> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------
SQL_ID? 97pta6n0k7wtv, child number 0

select * from v_emp where empno=:x

NOTE: cannot fetch plan for SQL_ID: 97pta6n0k7wtv, CHILD_NUMBER: 0
????? Please verify value of SQL_ID and CHILD_NUMBER;
????? It could also be that the plan is no longer in cursor cache (check v$sql_plan)

SCOTT@test> select sql_text from v$sql where sql_id='97pta6n0k7wtv';
SQL_TEXT
------------------------------------------------------------
select * from v_emp where empno=:x

--可以發現僅僅1個子光標。

4.如果在訪問時加入本地表看看：

select a.* from (select * from v_emp where empno=1) a,dual;
select a.* from (select * from v_emp where empno=2) a,dual;
select a.* from (select * from v_emp where empno=3) a,dual;

SCOTT@test> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID? 5puh2hnr82su0, child number 0
-------------------------------------
select a.* from (select * from v_emp where empno=:"SYS_B_0") a,dual
Plan hash value: 242706220
---------------------------------------------------------------------------------------
| Id? | Operation???? | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time?? | Inst?? |IN-OUT|
---------------------------------------------------------------------------------------
|?? 1 |? NESTED LOOPS |????? |????? 1 |??? 37 |???? 3?? (0)| 00:00:01 |??????? |????? |
|?? 2 |?? FAST DUAL?? |????? |????? 1 |?????? |???? 2?? (0)| 00:00:01 |??????? |????? |
|?? 3 |?? REMOTE????? | EMP? |????? 1 |??? 37 |???? 1?? (0)| 00:00:01 | LOOPB~ | R->S |
---------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
?? 1 - SEL$5C160134
?? 2 - SEL$5C160134 / DUAL@SEL$1
?? 3 - SEL$5C160134 / EMP@SEL$3

--可以發現child number 0，也沒有產生子光標。

--在生產系統由于產生大量子光標，導致出現cursor: pin S wait on X等待事件，最終導致出現ora-4031 錯誤，目前不得不重啟數據庫。

Errors in file /u01/app/oracle/admin/mid/bdump/mid_smon_3832.trc:
ORA-04031: Message 4031 not found; No message file for product=RDBMS, facility=ORA; arguments: [4064] [shared pool] [lock table sys.mon_mods$ in ...] [sga heap(1,0)] [kglsim heap]
ORA-04031: Message 4031 not found; No message file for product=RDBMS, facility=ORA; arguments: [3896] [shared pool] [select ts#,file#,block#,cols...] [sga heap(1,0)] [kglsim object batch]

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