在解析SQL語句的時候，如果在共享池中發現匹配的SQL語句，就可以避免掉解析的大部分開銷。在共享池中找到匹配的SQL語句所對應的解析被稱為軟解析(soft parse)。如果沒有找到匹配的SQL語句，則必須進行硬解析（hard parse）。

硬解析不僅耗費CPU時間，在有大量會話想要同時快取SQL語句到共享池時還會造成爭用。透過使用繫結變數，可以最小化解析的代價。 

1.CURSOR_SHARING引數

該引數轉換SQL語句中的字面值到繫結變數。轉換值提高了遊標共享，且可能會影響SQL語句的執行計劃。最佳化器是基於繫結變數的存在生成執行計劃，而不是實際字面量值。

CURSOR_SHARING決定什麼型別的SQL語句可以共享相同的遊標。CURSOR_SHARING引數有三個值：

FORCE：只要有可能，字面量就會被替換為繫結變數。SIMILAR：只有當替換不會影響到執行計劃時，才會將字面量替換為繫結變數EXACT：這是預設值。不將字面量替換為繫結變數。

注意：不推薦修改CURSOR_SHARING引數的預設值。如果實在無法修改現有應用的程式碼，可以透過設定CURSOR_SHARING引數來指示Oracle透明地將字面量替換為繫結變數。 

2.識別沒有使用繫結變數的SQL語句

利用v$sql檢視的FORCE_MATCHING_SIGNATURE欄位，可以識別可能從繫結變數或CURSOR_SHARING獲益的SQL語句。如果SQL已使用繫結變數或者CURSOR_SHARING,則FORCE_MATCHING_SIGNATURE在對其進行標識時將給出同樣的簽名。換句話說，如果兩個SQL語句除了字面量的值之外都是相同的，它們將擁有相同的FORCE_MATCHING_SIGNATURE,這意味著如果為他們提供了繫結變數或者CURSOR_SHARING,它們就成了完全相同的語句。

使用FORCE_MATCHING_SIGNATURE識別沒有使用繫結變來的SQL語句。

with force_mathces as
(select l.force_matching_signature mathces,
max(l.sql_id || l.child_number) max_sql_child,
dense_rank() over(order by count(*) desc) ranking
from v$sql l
where l.force_matching_signature <> 0
and l.parsing_schema_name <> 'SYS'
group by l.force_matching_signature
having count(*) > 10)
select v.sql_id, v.sql_text, v.parsing_schema_name, fm.mathces, fm.ranking
from force_mathces fm, v$sql v
where fm.max_sql_child = (v.sql_id || v.child_number)
and fm.ranking <= 5
order by fm.ranking; 

3.透過執行動態SQL語句獲取繫結變數的好處

透過執行動態SQL語句，比較字面量和繫結引數對SQL解析的影響。

declare
v_ename emp.ename%type;
v_sal emp.sal%type;
v_sql clob;
begin

dbms_output.put_line('*********使用字面量************');
for vrt_emp in (select * from emp) loop
v_sql := 'select e.ename,e.sal from emp e where e.empno =' ||
vrt_emp.empno;
execute immediate v_sql
into v_ename, v_sql;
dbms_output.put_line(v_ename || ':' || v_sql);
end loop;

dbms_output.put_line('');
dbms_output.put_line('*********使用繫結變數************');
for vrt_emp in (select * from emp) loop
v_sql := 'select e.ename,e.sal from emp e where e.empno =:empno';
execute immediate v_sql
into v_ename, v_sql
using vrt_emp.empno;
dbms_output.put_line(v_ename || ':' || v_sql);
end loop;

end;

查詢v$sql檢視，比較執行結果：

SQL> select v.sql_text, v.sql_id, v.force_matching_signature
2 from v$sql v
3 where v.sql_text like 'select e.ename,e.sal from emp e where e.empno %';

SQL_TEXT SQL_ID FORCE_MATCHING_SIGNATURE
--------------------------------------------------------- -------------- ------------------------
select e.ename,e.sal from emp e where e.empno =7782 766syjydcn5fh 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7499 6ymy4hcb386vt 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7934 3t96y707p8by7 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7902 f9pyzxf7tnuzw 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7654 fvk1fzmrvjc4j 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7788 gsmatg9f4jd2z 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7566 4q9pzzpvvdpuu 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7369 3xhqmvm5vdqy0 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7698 bjjjw0gzaprzv 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7876 8nd8v8mrzxw4w 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7521 5tnyy066zfk1b 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7844 4kd7jb013g2zz 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =7900 grx9sh4fwrcwx 1.27397653964533E19
select e.ename,e.sal from emp e where e.empno =:empno 20wmyr4cvrr6k 3.49355109645567E18
select e.ename,e.sal from emp e where e.empno =7839 6u2ajyu05gw9s 1.27397653964533E19 

在v$sql檢視中，發現使用字面量的SQL語句有14條，而使用繫結變數的SQL語句只有一條。其中使用字面量的SQL語句除以了字面量值不同之外，其他部分都是相同。而FORCE_MATCHING_SIGNATURE的值是在假設該SQL語句使用繫結變數或者CURSOR_SHARING得到的，因此透過FORCE_MATCHING_SIGNATURE欄位識別沒有繫結變數的SQL語句。

---實踐案例
select to_char(FORCE_MATCHING_SIGNATURE) as FORCE_MATCHING_SIGNATURE, count(1) as counts
from v$sql
where FORCE_MATCHING_SIGNATURE>0 and FORCE_MATCHING_SIGNATURE <> EXACT_MATCHING_SIGNATURE
group by FORCE_MATCHING_SIGNATURE
having count(1) > &a
order by 2 desc;

 

create table tb_force(id integer,name varchar2(10));
insert into tb_force(id,name) values ('1','111');
insert into tb_force(id,name) values ('2','222');
insert into tb_force(id,name) values ('3','333');
commit;

SQL> show parameter cursor_sharing;
 
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cursor_sharing                       string      EXACT

 
SQL> select /*mystatement*/ * from tb_force where id=1;
 
                                     ID NAME
--------------------------------------- ----------
                                      1 111
SQL> select /*mystatement*/ * from tb_force where id=2;
 
                                     ID NAME
--------------------------------------- ----------
                                      2 222
SQL> select /*mystatement*/ * from tb_force where id=3;
 
                                     ID NAME
--------------------------------------- ----------
                                      3 333

SQL> select sql_text,FORCE_MATCHING_SIGNATURE,EXACT_MATCHING_SIGNATURE from v$sql t where sql_text like '%mystatement%';
 
SQL_TEXT                                                                         FORCE_MATCHING_SIGNATURE EXACT_MATCHING_SIGNATURE
-------------------------------------------------------------------------------- ------------------------ ------------------------
 select /*mystatement*/ * from tb_force where id=2                                    1.30958144015941E19      6.87046715690462E18
 select /*mystatement*/ * from tb_force where id=1                                    1.30958144015941E19      5.16419277214989E17
 select /*mystatement*/ * from tb_force where id=3                                    1.30958144015941E19      1.36147732398326E19
--id取不同值時，FORCE_MATCHING_SIGNATURE的值相同，而EXACT_MATCHING_SIGNATURE值不同，說明在cursor_sharing=force時，執行計劃可共用；cursor_sharing=exact時，執行計劃不可共用；實際

上我們使用繫結變數後，同樣也可以達到執行計劃共用的效果。

> alter system set cursor_sharing=force;

System altered.

> show parameter cursor_sharing;

NAME         TYPE  VALUE
------------------------------------ ----------- ------------------------------
cursor_sharing        string  FORCE

> alter system flush shared_pool;

System altered.

SQL> select /*mystatement*/ * from tb_force where id=1;
 
                                     ID NAME
--------------------------------------- ----------
                                      1 111
 
SQL> select /*mystatement*/ * from tb_force where id=2;
 
                                     ID NAME
--------------------------------------- ----------
                                      2 222
 
SQL> select /*mystatement*/ * from tb_force where id=3;
 
                                     ID NAME
--------------------------------------- ----------
                                      3 333

SQL> select sql_text,FORCE_MATCHING_SIGNATURE,EXACT_MATCHING_SIGNATURE from v$sql t where sql_text like '%mystatement%';
 
SQL_TEXT                                                                         FORCE_MATCHING_SIGNATURE EXACT_MATCHING_SIGNATURE
-------------------------------------------------------------------------------- ------------------------ ------------------------
 select /*mystatement*/ * from tb_force where id=:"SYS_B_0"                           1.30958144015941E19      1.30958144015941E19
--當我們把cursor_sharing設定為force後，發現語句執行計劃共用了，並且自動使用繫結變數(此時v$sql檢視中只查詢到一條與之相關的語句)。

--透過上面的實驗，我們發現查詢系統中有哪些語句未使用繫結變數，可根據FORCE_MATCHING_SIGNATURE實現。

--下面我們就透過FORCE_MATCHING_SIGNATURE來查詢系統中未使用繫結變數的語句...
select to_char(FORCE_MATCHING_SIGNATURE) as FORCE_MATCHING_SIGNATURE, count(1) as counts
from v$sql
where FORCE_MATCHING_SIGNATURE>0 and FORCE_MATCHING_SIGNATURE <> EXACT_MATCHING_SIGNATURE
group by FORCE_MATCHING_SIGNATURE
having count(1) > &a
order by 2 desc;

--透過上面查詢出來的FORCE_MATCHING_SIGNATURE值，來查詢sql語句
select t.* from v$sql t where FORCE_MATCHING_SIGNATURE=16456394970215993993;

--延伸：
cursor_sharing有三個可選引數：EXACT、SIMILAR、FORCE。

--語句完全一樣，則利用share pool中的語句，反之則重新生成(預設)。
EXACT：Only allows statements with identical text to share the same cursor.

--語句相似，但系統自行判斷是否利用share pool中的語句(不推薦，bug很多)。
SIMILAR：Causes statements that may differ in some literals, but are otherwise identical, to share a cursor, unless the literals affect either the meaning of the statement or

the degree to which the plan is optimized.

--語句相似，系統強制利用share pool中的語句(不推薦，bug較多)。
FORCE：Forces statements that may differ in some literals, but are otherwise identical, to share a cursor, unless the literals affect the meaning of the statement.

 

參考連線：http://blog.csdn.net/orcldb/article/details/9041139