怎么理解数据库的初始化参数cursor_sharing

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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。

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：

----首先创建一张woo表----

WOO@woo> create table woo (id int,name varchar2(10));Table created.Elapsed: 00:00:00.06

---产生一些数据----

WOO@woo> insert into woo values(1,'aa');1 row created.Elapsed: 00:00:00.00WOO@woo> insert into woo values(2,'bb');1 row created.Elapsed: 00:00:00.00WOO@woo> insert into woo values(3,'cc');1 row created.Elapsed: 00:00:00.00WOO@woo> insert into woo values (4,'dd');1 row created.Elapsed: 00:00:00.00WOO@woo> commit;Commit complete.Elapsed: 00:00:00.00WOO@woo> select * from woo;        ID NAME---------- ----------         1 aa         2 bb         3 cc         4 ddElapsed: 00:00:00.01

---创建下面实验将要用到的三张表----

WOO@woo> create table woo_exact as select * from woo;Table created.Elapsed: 00:00:00.01WOO@woo> create table woo_similar as select * from woo;Table created.Elapsed: 00:00:00.01WOO@woo> create table woo_force as select * from woo;Table created.Elapsed: 00:00:00.00

---查看当前session的trace文件的路径----

WOO@woo> SELECT d.Value || '/' || Lower(Rtrim(i.Instance, Chr(0))) || '_ora_' ||  2         p.Spid || '.trc' AS "trace_file_name"  3    FROM (SELECT p.Spid  4            FROM V$mystat m, V$session s, V$process p  5           WHERE m.Statistic# = 1  6             AND s.Sid = m.Sid  7             AND p.Addr = s.Paddr) p,  8         (SELECT t.Instance  9            FROM V$thread t, V$parameter v 10           WHERE v.Name = 'thread' 11             AND (v.Value = 0 OR t.Thread# = To_Number(v.Value))) i, 12         (SELECT VALUE 13            FROM V$parameter 14           WHERE NAME = 'user_dump_dest') d;trace_file_name-------------------------------------------------------/DBSoft/diag/rdbms/woo/woo/trace/woo_ora_37746.trcElapsed: 00:00:00.01

2.2 cursor_sharing=exact的情况：

WOO@woo>  alter session set cursor_sharing=exact;Session altered.Elapsed: 00:00:00.00WOO@woo>  alter session set sql_trace=true;Session altered.Elapsed: 00:00:00.00WOO@woo> select * from woo_exact where id=1;        ID NAME---------- ----------         1 aaElapsed: 00:00:00.00WOO@woo> select * from woo_exact where id=2;        ID NAME---------- ----------         2 bbElapsed: 00:00:00.01WOO@woo> select * from woo_exact where id=3;        ID NAME---------- ----------         3 ccElapsed: 00:00:00.00WOO@woo> select * from woo_exact where id=1;        ID NAME---------- ----------         1 aaElapsed: 00:00:00.00

----从下面的查询可以看出执行了两次硬解析----

WOO@woo> select sql_text from v$sql where sql_text like 'select * from woo_exact where%';SQL_TEXT---------------------------------------------------------------------------------------select * from woo_exact where id=1select * from woo_exact where id=3select * from woo_exact where id=2Elapsed: 00:00:00.05NAME                                                                  VALUE---------------------------------------------------------------- ----------ADG parselock X get attempts                                              0ADG parselock X get successes                                             0parse time cpu                                                          326parse time elapsed                                                      307parse count (total)                                                   56211parse count (hard)                                                     1681parse count (failures)                                                   10parse count (describe)                                                    08 rows selected.

cursor_sharing=similar的情况：

WOO@woo> alter session set cursor_sharing=similar;Session altered.Elapsed: 00:00:00.00WOO@woo> alter system flush shared_pool;System altered.Elapsed: 00:00:00.13WOO@woo> select * from woo_similar where id=1;        ID NAME---------- ----------         1 aaElapsed: 00:00:00.01WOO@woo> select * from woo_similar where id=4;        ID NAME---------- ----------         4 ddElapsed: 00:00:00.00WOO@woo> select * from woo_similar where id=8;no rows selectedElapsed: 00:00:00.00

----在这里可以看到执行两次SQL查询，只进行了一个硬解析----

WOO@woo> select sql_text from v$sql where sql_text like 'select * from woo_similar where %';SQL_TEXT--------------------------------------------------------------------------------------------------------select * from woo_similar where id=:"SYS_B_0"Elapsed: 00:00:00.02WOO@woo> select name,value from v$sysstat where name like '%parse%'; NAME                                                                  VALUE---------------------------------------------------------------- ----------ADG parselock X get attempts                                              0ADG parselock X get successes                                             0parse time cpu                                                          374parse time elapsed                                                      352parse count (total)                                                   57024parse count (hard)                                                     2006parse count (failures)                                                   10parse count (describe)                                                    08 rows selected.Elapsed: 00:00:00.00WOO@woo>

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。

上面的例子还不能足以说明该情况，接着下面的模拟：

cursor_sharing=force的情况

WOO@woo> alter session set cursor_sharing=force;Session altered.Elapsed: 00:00:00.00WOO@woo> alter system flush shared_pool;System altered.Elapsed: 00:00:00.07 WOO@woo> alter session set sql_trace=true;Session altered.Elapsed: 00:00:00.02WOO@woo> select * from woo_force where id=1;        ID NAME---------- ----------         1 aaElapsed: 00:00:00.00WOO@woo> select * from woo_force where id=4;        ID NAME---------- ----------         4 ddElapsed: 00:00:00.00WOO@woo> select * from woo_force where id=1;        ID NAME---------- ----------         1 aaElapsed: 00:00:00.00

----从下面的查询中可以看出只进行了一次硬解析，而且使用了绑定变量----

WOO@woo>  select sql_text from v$sql where sql_text like 'select * from woo_force where %';SQL_TEXT-------------------------------------------------------------------------------------------select * from woo_force where id=:"SYS_B_0"Elapsed: 00:00:00.02WOO@woo> select name,value from v$sysstat where name like '%parse%'; NAME                                                                  VALUE---------------------------------------------------------------- ----------ADG parselock X get attempts                                              0ADG parselock X get successes                                             0parse time cpu                                                          379parse time elapsed                                                      355parse count (total)                                                   57385parse count (hard)                                                     2145parse count (failures)                                                   10parse count (describe)                                                    08 rows selected.Elapsed: 00:00:00.01

总结：force是在任何情况下，无条件重用SQL。

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