Oracle參數之cursor_sharing

1、Cursor_sharing簡介：
這個參數是用來告訴Oracle在什麼狀況下能夠共享遊標，即SQL重用。
Cursor_sharing參數有3個值能夠設置：
1)、EXACT：一般來講，exact值是Oracle推薦的，也是默認的，它要求SQL語句在徹底相同時纔會重用，不然會被從新執行硬解析操做。
2)、SIMILAR：similar是在Oracle認爲某條SQL語句的謂詞條件可能會影響到它的執行計劃時，纔會被從新分析，不然將重用SQL。
3)、FORCE：force是在任何狀況下，無條件重用SQL。
4)、在Oracle12c版本之後，不建議設置成SIMILAR。手冊已經把該參數廢棄。
備註：上面所說的SQL重用，僅僅是指謂詞條件不一樣的SQL語句，實際上這樣的SQL基本上都在執行一樣的業務操做。
2、在Cursor_sharing參數值不一樣的時對SQL的影響：
2.1 建立實驗環境：
11G：建立基表，錄入數據。
SYS@orcl> create table test (id number,name varchar2(10));
SYS@orcl> insert into test values (1,'aa');
SYS@orcl> insert into test values (2,'bb');
SYS@orcl> insert into test values (3,'cc');
SYS@orcl> commit;

建立三張實驗用表：
SYS@orcl> create table test_exact as select from test;
SYS@orcl> create table test_similar as select from test;
SYS@orcl> create table test_force as select * from test;

設置跟蹤trace
SYS@orcl> oradebug setmypid
SYS@orcl> oradebug tracefile_name

測試exact：
SYS@orcl> alter session set cursor_sharing=exact;
SYS@orcl> alter session set sql_trace=true;
SYS@orcl> select * from test_exact where id=1;
ID NAME

---

1 aa

SYS@orcl> select * from test_exact where id=2;

ID NAME

---

2 bb

SYS@orcl> select * from test_exact where id=3;

ID NAME

---

3 cc

SYS@orcl> alter session set sql_trace=false;

觀察trace文件：
SYS@orcl> select sql_text from v$sql where sql_text like 'select * from test_ex%';
SQL_TEXT

select from test_exact where id=2
select from test_exact where id=3
select * from test_exact where id=1

[oracle@orcl ~]$ tkprof /u01/app/oracle/diag/rdbms/orcl/orcl/trace/orcl_ora_2849.trc out.txt aggregate=no

SQL ID: 22cdhbrvt2nmw
Plan Hash: 3210958934
select *
from
test_exact where id=1
call count cpu elapsed disk query current rows

---

Parse 1 0.00 0.01 2 2 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 0 4 0 1

---

total 4 0.00 0.01 2 6 0 1

Misses in library cache during parse: 1 --硬解析1次
Optimizer mode: ALL_ROWS
Parsing user id: SYS

SQL ID: f9kq2n9utcww7
Plan Hash: 3210958934
select *
from
test_exact where id=2
call count cpu elapsed disk query current rows

---

Parse 1 0.00 0.00 0 1 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 0 4 0 1

---

total 4 0.00 0.00 0 5 0 1

Misses in library cache during parse: 1 --硬解析1次
Optimizer mode: ALL_ROWS
Parsing user id: SYS

SQL ID: 22cdhbrvt2nmw
Plan Hash: 3210958934
select *
from
test_exact where id=1
call count cpu elapsed disk query current rows

---

Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 0 4 0 1

---

total 4 0.00 0.00 0 4 0 1
Misses in library cache during parse: 0 --軟解析
Optimizer mode: ALL_ROWS
Parsing user id: SYS
總結：當cursor_sharing=exact時，只有當SQL語句是徹底同樣的狀況下才能被重用。

測試SIMILAR：

SYS@orcl> oradebug setmypid
SYS@orcl> oradebug tracefile_name
SYS@orcl> alter session set cursor_sharing=similar;
SYS@orcl> alter session set sql_trace=true;
SYS@orcl> select * from test_similar where id=1;
ID NAME

---

1 aa

SYS@orcl> select * from test_similar where id=2;
ID NAME

---

2 bb

SYS@orcl> select from test_similar where id=10;
no rows selected
SYS@orcl> select sql_text from v$sql where sql_text like 'select from test_similar where%';
SQL_TEXT

select from test_similar where id=:"SYS_B_0"
select from test_similar where id=:"SYS_B_0"
select * from test_similar where id=:"SYS_B_0"
SYS@orcl> alter session set sql_trace=false;

分析trace文件：
SQL ID: 6wvc0ymwz50uq
Plan Hash: 3269012161
select *
from
test_similar where id=:"SYS_B_0"

call count cpu elapsed disk query current rows

---

Parse 3 0.00 0.00 1 1 0 0
Execute 3 0.00 0.00 1 3 0 0
Fetch 5 0.00 0.00 0 11 0 2

---

total 11 0.00 0.00 2 15 0 2

Misses in library cache during parse: 3 --執行三次硬解析
Optimizer mode: ALL_ROWS
Parsing user id: SYS

對於SIMILAR的狀況，若是CBO發現被綁定變量的謂詞還有其餘的執行計劃能夠選擇時，若是謂詞條件的值有變化，就將會產生一個新的子游標，而不是重用以前的SQL；若是謂詞沒有其餘的執行計劃可選擇，則忽略謂詞的值，重用以前的SQL。

進一步測試：

SYS@orcl> alter system flush shared_pool;
SYS@orcl> alter system flush buffer_cache;
SYS@orcl> insert into test_similar values (1,'abc');
SYS@orcl> commit;
SYS@orcl> create index ind_test_similar on test_similar(id);
SYS@orcl> exec dbms_stats.gather_table_stats(user,'test_similar',cascade=>true);
SYS@orcl> alter session set cursor_sharing=similar;
SYS@orcl> alter session set sql_trace=true;
SYS@orcl> select * from test_similar where id=1 and name='aa';
ID NAME

---

1 aa

SYS@orcl> select * from test_similar where id=1 and name='abc';
ID NAME

---

1 abc

SYS@orcl> alter session set sql_trace=false;
SYS@orcl> select sql_text from v$sql where sql_text like 'select * from test_similar where%';
SQL_TEXT

select * from test_similar where id=:"SYS_B_0" and name=:"SYS_B_1

查看trace：
SQL ID: 23gux1agm4fnt
Plan Hash: 3269012161
select *
from
test_similar where id=:"SYS_B_0" and name=:"SYS_B_1"

call count cpu elapsed disk query current rows

---

Parse 2 0.00 0.00 0 0 0 0
Execute 2 0.00 0.00 0 0 0 0
Fetch 4 0.00 0.00 0 9 0 2

---

total 8 0.00 0.00 0 9 0 2

Misses in library cache during parse: 1 --硬解析1次
Optimizer mode: ALL_ROWS
Parsing user id: SYS

測試FORCE：
SYS@orcl> oradebug setmypid
SYS@orcl> oradebug tracefile_name
/u01/app/oracle/diag/rdbms/orcl/orcl/trace/orcl_ora_4104.trc
SYS@orcl> alter session set cursor_sharing=force;
SYS@orcl> alter session set sql_trace=true;
SYS@orcl> select * from test_force where id=1;
ID NAME

---

1 aa

SYS@orcl> select * from test_force where id=2;
ID NAME

---

2 bb

SYS@orcl> select * from test_force where id=1;
ID NAME

---

1 aa

SYS@orcl> alter session set sql_trace=false;
SYS@orcl> select sql_text from v$sql where sql_text like 'select * from test_force where%';
SQL_TEXT

select * from test_force where id=:"SYS_B_0"

查看trace：
SQL ID: 5my70999m011j
Plan Hash: 1419416768
select *
from
test_force where id=:"SYS_B_0"

call count cpu elapsed disk query current rows

---

Parse 3 0.00 0.00 1 1 0 0
Execute 3 0.00 0.00 1 1 0 0
Fetch 6 0.00 0.00 0 12 0 3

---

total 12 0.00 0.00 2 14 0 3

Misses in library cache during parse: 1Optimizer mode: ALL_ROWSParsing user id: SYS總結：force是在任何狀況下，無條件重用SQL。