Oracle參數之cursor_sharing

一、Cursor_sharing簡介：
這個參數是用來告訴Oracle在什么情況下可以共享游標，即SQL重用。
Cursor_sharing參數有3個值可以設置：
1)、EXACT：通常來說，exact值是Oracle推薦的，也是默認的，它要求SQL語句在完全相同時才會重用，否則會被重新執行硬解析操作。
2)、SIMILAR：similar是在Oracle認為某條SQL語句的謂詞條件可能會影響到它的執行計劃時，才會被重新分析，否則將重用SQL。
3)、FORCE：force是在任何情況下，無條件重用SQL。
4)、在Oracle12c版本以后，不建議設置成SIMILAR。手冊已經把該參數廢棄。
備注：上面所說的SQL重用，僅僅是指謂詞條件不同的SQL語句，實際上這樣的SQL基本上都在執行同樣的業務操作。
二、在Cursor_sharing參數值不同的時對SQL的影響：
2.1 創建實驗環境：
11G：創建基表，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;

創建三張實驗用表：
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語句是完全一樣的情況下才能被重用。

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

對于SIMILAR的情況，如果CBO發現被綁定變量的謂詞還有其他的執行計劃可以選擇時，如果謂詞條件的值有變化，就將會產生一個新的子游標，而不是重用之前的SQL；如果謂詞沒有其他的執行計劃可選擇，則忽略謂詞的值，重用之前的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: 1
Optimizer mode: ALL_ROWS
Parsing user id: SYS
總結：force是在任何情況下，無條件重用SQL。