一、Cursor_sharing简介：
这个参数是用来告诉Oracle在什么情况下可以共享游标，即SQL重用。
Cursor_sharing参数有3个值可以设置：
1)、EXACT：通常来说，exact值是Oracle推荐的，也是默认的，它要求SQL语句在完全相同时才会重用，否则会被重新执行硬解析操作。
2)、SIMILAR：similar是在Oracle认为某条SQL语句的谓词条件可能会影响到它的执行计划时，才会被重新分析，否则将重用SQL。
3)、FORCE：force是在任何情况下，无条件重用SQL。
4)、在Oracle12c版本以后，不建议设置成SIMILAR。手册已经把该参数废弃。
备注：上面所说的SQL重用，仅仅是指谓词条件不同的SQL语句，实际上这样的SQL基本上都在执行同样的业务操作。
二、在Cursor_sharing参数值不同的时对SQL的影响：
2.1 创建实验环境：
11G：创建基表，录入数据。
[email protected]> create table test (id number,name varchar2(10));
[email protected]> insert into test values (1,‘aa‘);
[email protected]> insert into test values (2,‘bb‘);
[email protected]> insert into test values (3,‘cc‘);
[email protected]> commit;
创建三张实验用表：
[email protected]> create table test_exact as select from test;
[email protected]> create table test_similar as select from test;
[email protected]> create table test_force as select * from test;
设置跟踪trace
[email protected]> oradebug setmypid
[email protected]> oradebug tracefile_name
测试exact：
[email protected]> alter session set cursor_sharing=exact;
[email protected]> alter session set sql_trace=true;
[email protected]> select * from test_exact where id=1;
ID NAME
1 aa[email protected]> select * from test_exact where id=2;
ID NAME 2 bb[email protected]> select * from test_exact where id=3;
ID NAME 3 cc[email protected]> alter session set sql_trace=false;
观察trace文件：
[email protected]> select sql_text from v$sql where sql_text like ‘select * from test_ex%‘;
SQL_TEXT
select from test_exact where id=2
select from test_exact where id=3
select * from test_exact where id=1
[[email protected] ~]$ 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 --软解析
Optimizer mode: ALL_ROWS
Parsing user id: SYS
总结：当cursor_sharing=exact时，只有当SQL语句是完全一样的情况下才能被重用。
测试SIMILAR：
[email protected]> oradebug setmypid
[email protected]> oradebug tracefile_name
[email protected]> alter session set cursor_sharing=similar;
[email protected]> alter session set sql_trace=true;
[email protected]> select * from test_similar where id=1;
ID NAME
1 aa[email protected]> select * from test_similar where id=2;
ID NAME
2 bb
[email protected]> select from test_similar where id=10;
no rows selected
[email protected]> select sql_text from v$sql where sql_text like ‘select from test_similar where%‘;
SQL_TEXT
select from test_similar where id=:"SYS_B_0"
select from test_similar where id=:"SYS_B_0"
select * from test_similar where id=:"SYS_B_0"
[email protected]> alter session set sql_trace=false;
分析trace文件：
SQL ID: 6wvc0ymwz50uq
Plan Hash: 3269012161
select *
from
test_similar where id=:"SYS_B_0"
call count cpu elapsed disk query current rows
Parse 3 0.00 0.00 1 1 0 0
Execute 3 0.00 0.00 1 3 0 0
Fetch 5 0.00 0.00 0 11 0 2
total 11 0.00 0.00 2 15 0 2
Misses in library cache during parse: 3 --执行三次硬解析
Optimizer mode: ALL_ROWS
Parsing user id: SYS
对于SIMILAR的情况，如果CBO发现被绑定变量的谓词还有其他的执行计划可以选择时，如果谓词条件的值有变化，就将会产生一个新的子游标，而不是重用之前的SQL；如果谓词没有其他的执行计划可选择，则忽略谓词的值，重用之前的SQL。
进一步测试：
[email protected]> alter system flush shared_pool;
[email protected]> alter system flush buffer_cache;
[email protected]> insert into test_similar values (1,‘abc‘);
[email protected]> commit;
[email protected]> create index ind_test_similar on test_similar(id);
[email protected]> exec dbms_stats.gather_table_stats(user,‘test_similar‘,cascade=>true);
[email protected]> alter session set cursor_sharing=similar;
[email protected]> alter session set sql_trace=true;
[email protected]> select * from test_similar where id=1 and name=‘aa‘;
ID NAME
1 aa[email protected]> select * from test_similar where id=1 and name=‘abc‘;
ID NAME
1 abc
[email protected]> alter session set sql_trace=false;
[email protected]> select sql_text from v$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
测试FORCE：
[email protected]> oradebug setmypid
[email protected]> oradebug tracefile_name
/u01/app/oracle/diag/rdbms/orcl/orcl/trace/orcl_ora_4104.trc
[email protected]> alter session set cursor_sharing=force;
[email protected]> alter session set sql_trace=true;
[email protected]> select * from test_force where id=1;
ID NAME
1 aa[email protected]> select * from test_force where id=2;
ID NAME
2 bb[email protected]> select * from test_force where id=1;
ID NAME
1 aa
[email protected]> alter session set sql_trace=false;
[email protected]> 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
总结：force是在任何情况下，无条件重用SQL。