[20160302]繫結變數的分配長度2.txt

[20160302]繫結變數的分配長度2.txt

--如果繫結變數中字串分配佔用空間的長度變化，oracle會建立子游標。
--參考連線：
http://blog.itpub.net/267265/viewspace-1993495/

--oracle 可以透過一個10503事件設定大的快取，測試看看：
$ oerr ora 10503
10503, 00000, "enable user-specified graduated bind lengths"
// *Cause:
// *Action:

1.環境：

SCOTT@book> @  &r/ver1

PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

create table t (c1 varchar2(4000));

-- ALTER SESSION SET EVENTS '10503 trace name context level <buffer length>, forever';
ALTER SESSION SET EVENTS '10503 trace name context level 2000, forever';

declare
v_c1 varchar2(4000);
  begin
   for i in 1..4000 loop
     v_c1 := rpad('X',i);
     execute immediate 'insert into t values (:instring) ' using v_c1 ;
    end loop;
end;
/
commit;

declare
v_c1 varchar2(4000);
  begin
   for i in 1..4000 loop
     v_c1 := rpad('X',i);
     execute immediate 'select count(*) from t where c1=:instring' using v_c1 ;
    end loop;
end;
/
commit;

--確定sql_id='9mrd273576n14'

SCOTT@book> select sql_id, child_number, executions  from v$sql where sql_id = '9mrd273576n14';
SQL_ID        CHILD_NUMBER EXECUTIONS
------------- ------------ ----------
9mrd273576n14            0       2000
9mrd273576n14            1       2000

SCOTT@book> @ &r/bind_cap 9mrd273576n14
C200
------------------------------------------
select count(*) from t where c1=:instring

SQL_ID        CHILD_NUMBER WAS NAME                   POSITION MAX_LENGTH LAST_CAPTURED       DATATYPE_STRING VALUE_STRING
------------- ------------ --- -------------------- ---------- ---------- ------------------- --------------- -------------
9mrd273576n14            0 YES :INSTRING                     1       2000 2016-03-02 15:39:23 VARCHAR2(2000)  X
                         1 YES :INSTRING                     1       4000 2016-03-02 15:39:23 VARCHAR2(4000)  X

--可以發現這樣僅僅存在2個游標。

SCOTT@book> alter system flush shared_pool;
System altered.

SCOTT@book> ALTER SESSION SET EVENTS '10503 trace name context  off';
Session altered.

--重新執行看看。
--也可以退出再登入執行看看。

SCOTT@book> select sql_id, child_number, executions  from v$sql where sql_id = '9mrd273576n14';

SQL_ID        CHILD_NUMBER EXECUTIONS
------------- ------------ ----------
9mrd273576n14            0         32
9mrd273576n14            1         96
9mrd273576n14            2       1872
9mrd273576n14            3       2000

--這種情況僅僅存在這種因素產生大量子游標的情況下可以考慮這種方式。