本篇内容介绍了“怎么理解数据库的初始化参数cursor_sharing”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
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: 00:00:00.00 WOO@woo> insert into woo values(2,'bb'); 1 row created. Elapsed: 00:00:00.00 WOO@woo> insert into woo values(3,'cc'); 1 row created. Elapsed: 00:00:00.00 WOO@woo> insert into woo values (4,'dd'); 1 row created. Elapsed: 00:00:00.00 WOO@woo> commit; Commit complete. Elapsed: 00:00:00.00 WOO@woo> select * from woo; ID NAME ---------- ---------- 1 aa 2 bb 3 cc 4 dd Elapsed: 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: 00:00:00.01 WOO@woo> create table woo_similar as select * from woo; Table created. Elapsed: 00:00:00.01 WOO@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.trc Elapsed: 00:00:00.01
2.2 cursor_sharing=exact的情况：
WOO@woo> alter session set cursor_sharing=exact; Session altered. Elapsed: 00:00:00.00 WOO@woo> alter session set sql_trace=true; Session altered. Elapsed: 00:00:00.00 WOO@woo> select * from woo_exact where id=1; ID NAME ---------- ---------- 1 aa Elapsed: 00:00:00.00 WOO@woo> select * from woo_exact where id=2; ID NAME ---------- ---------- 2 bb Elapsed: 00:00:00.01 WOO@woo> select * from woo_exact where id=3; ID NAME ---------- ---------- 3 cc Elapsed: 00:00:00.00 WOO@woo> select * from woo_exact where id=1; ID NAME ---------- ---------- 1 aa Elapsed: 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$sql where sql_text like 'select * from woo_exact where%'; SQL_TEXT --------------------------------------------------------------------------------------- select * from woo_exact where id=1 select * from woo_exact where id=3 select * from woo_exact where id=2 Elapsed: 00:00:00.05 NAME VALUE ---------------------------------------------------------------- ---------- ADG parselock X get attempts 0 ADG parselock X get successes 0 parse time cpu 326 parse time elapsed 307 parse count (total) 56211 parse count (hard) 1681 parse count (failures) 10 parse count (describe) 0 8 rows selected.
cursor_sharing=similar的情况：
WOO@woo> alter session set cursor_sharing=similar; Session altered. Elapsed: 00:00:00.00 WOO@woo> alter system flush shared_pool; System altered. Elapsed: 00:00:00.13 WOO@woo> select * from woo_similar where id=1; ID NAME ---------- ---------- 1 aa Elapsed: 00:00:00.01 WOO@woo> select * from woo_similar where id=4; ID NAME ---------- ---------- 4 dd Elapsed: 00:00:00.00 WOO@woo> select * from woo_similar where id=8; no rows selected Elapsed: 00:00:00.00
—-在这里可以看到执行两次SQL查询，只进行了一个硬解析—-
WOO@woo> select sql_text from v$sql where sql_text like 'select * from woo_similar where %'; SQL_TEXT -------------------------------------------------------------------------------------------------------- select * from woo_similar where id=:"SYS_B_0" Elapsed: 00:00:00.02 WOO@woo> select name,value from v$sysstat where name like '%parse%'; NAME VALUE ---------------------------------------------------------------- ---------- ADG parselock X get attempts 0 ADG parselock X get successes 0 parse time cpu 374 parse time elapsed 352 parse count (total) 57024 parse count (hard) 2006 parse count (failures) 10 parse count (describe) 0 8 rows selected. Elapsed: 00:00:00.00 WOO@woo>
对于SIMILAR的情况，如果CBO发现被绑定变量的谓词还有其他的执行计划可以选择时，如果谓词条件的值有变化，就将会产生一个新的子游标，而不是重用之前的SQL；如果谓词没有其他的执行计划可选择，则忽略谓词的值，重用之前的SQL。
上面的例子还不能足以说明该情况，接着下面的模拟：
cursor_sharing=force的情况
WOO@woo> alter session set cursor_sharing=force; Session altered. Elapsed: 00:00:00.00 WOO@woo> alter system flush shared_pool; System altered. Elapsed: 00:00:00.07 WOO@woo> alter session set sql_trace=true; Session altered. Elapsed: 00:00:00.02 WOO@woo> select * from woo_force where id=1; ID NAME ---------- ---------- 1 aa Elapsed: 00:00:00.00 WOO@woo> select * from woo_force where id=4; ID NAME ---------- ---------- 4 dd Elapsed: 00:00:00.00 WOO@woo> select * from woo_force where id=1; ID NAME ---------- ---------- 1 aa Elapsed: 00:00:00.00
—-从下面的查询中可以看出只进行了一次硬解析，而且使用了绑定变量—-
WOO@woo> select sql_text from v$sql where sql_text like 'select * from woo_force where %'; SQL_TEXT ------------------------------------------------------------------------------------------- select * from woo_force where id=:"SYS_B_0" Elapsed: 00:00:00.02 WOO@woo> select name,value from v$sysstat where name like '%parse%'; NAME VALUE ---------------------------------------------------------------- ---------- ADG parselock X get attempts 0 ADG parselock X get successes 0 parse time cpu 379 parse time elapsed 355 parse count (total) 57385 parse count (hard) 2145 parse count (failures) 10 parse count (describe) 0 8 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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