Oracle cursor_sharing 参数 详解

Oracle cursor_sharing 参数 详解

 

一.官网的说明

http://download.oracle.com/docs/cd/E11882_01/server.112/e17110/initparams042.htm#REFRN10025

1.1 CURSOR_SHARING

Property	Description
Parameter type	String
Syntax	CURSOR_SHARING = { SIMILAR | EXACT | FORCE }
Default value	EXACT
Modifiable	ALTER SESSION, ALTER SYSTEM
Basic	No

CURSOR_SHARING determines what kind of SQL statements can share the same cursors.

Values:

（1）FORCE

Allows the creation of a new cursor if sharing an existing cursor, or if the cursor plan is not optimal.

（2）SIMILAR

Causes statements that may differ in some literals, but are otherwise identical, to share a cursor, unless the literals affect either the meaning of the statement or the degree to which the plan is optimized.

（3）EXACT

Only allows statements with identical text to share the same cursor.

--只有SQL语句完全相同的情况下，才会使用相同的cursor，即执行计划。

Notes:

（1）If you set CURSOR_SHARING, then Oracle recommends the FORCE setting unless you are in a DSS environment. FORCE limits the growth of child cursors that can occur when the setting is SIMILAR.

（2）The value of the CURSOR_SHARING parameter has performance implications. Refer toOracle Database Performance Tuning Guidebefore setting this parameter.

1.2 When to Set CURSOR_SHARING to a Nondefault Value

The best practice is to write sharable SQL and use the default of EXACT for CURSOR_SHARING. However, for applications with many similar statements, setting CURSOR_SHARING can significantly improve cursor sharing, resulting in reduced memory usage, faster parses, and reduced latch contention. Consider this approach when statements in the shared pool differ only in the values of literals, and when response time is poor because of a very high number of library cache misses.

Setting CURSOR_SHARING to FORCE or SIMILAR has the following drawbacks:

（1）The database must perform extra work during the soft parse to find a similar statement in the shared pool.

（2）There is an increase in the maximum lengths (as returned by DESCRIBE) of any selected expressions that contain literals in a SELECT statement. However, the actual length of the data returned does not change.

（3）Star transformation is not supported.

（4）If stored outlines were generated with CURSOR_SHARING set to EXACT, then the database does not use stored outlines generated with literals. To avoid this problem, generate outlines with CURSOR_SHARING set to FORCE or SIMILAR and use the CREATE_STORED_OUTLINES parameter.

When deciding whether to set CURSOR_SHARING to FORCE or SIMILAR,consider the performance implications of each setting.

When CURSOR_SHARING is set to FORCE,the database uses one parent cursor and one child cursor for each distinct SQL statement.The database uses the same plan for each execution of the same statement.

When set to SIMILAR, database behavior depends on the presence of histograms:

（1）Histogram absent for column with system-generated bind value

Only one parent cursor and one child cursor exists for each distinct SQL statement. In this case, all executions of a SQL statement use the same plan.

（2）Histogram present for column with system-generated bind value

If the same SQL statement is executed multiple times, each execution has its own child cursor. In this case, the database peeks at bind variable values and create a new child cursor for each distinct value. Thus, each statement execution uses a plan based on the specific literals in the statement.

For example, consider the following statement:

SELECT * FROM hr.employees WHERE employee_id = 101

If FORCE is used, or if SIMILAR is used when no histogram exists,then the database optimizes this statement as if it contained a bind variable and uses bind peeking to estimate cardinality. Statements that differ only in the bind variable share the same execution plan.

If SIMILAR is used, and if a histogram does exist, then the database does not treat the statement as if a bind variable were used.The same query for a different employee may not use the same plan.

If you set CURSOR_SHARING,then Oracle recommends the FORCE setting unless you are in a DSS environment.FORCE limits the growth of child cursors that can occur when the setting is SIMILAR.

Also,function-based indexes may not work when using SIMILARbecause the database converts index parameters to bind variables.

For example, if the index is SUBSTR(id,1,3), then the database converts it to SUBSTR("ID",:SYS_B_0,:SYS_B_1)=:id,rendering the index invalid.

Oracle绑定变量详解

http://blog.csdn.net/xujinyang/article/details/6832627

二.测试

2.1 cursor_sharing=exact，这是cursor_sharing的默认值

2.1.1查看cursor_sharing值

SYS@anqing2(rac2)> show parameter cursor_sharing

NAMETYPEVALUE

------------------------- -------------------- ---------------------

cursor_sharingstringEXACT

2.1.2查看当前硬解析值

SYS@anqing2(rac2)>select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882056

parse time elapsed2648194

parse count (total)12780229

parse count (hard)9890010（硬解析次数）

parse count (failures)71

2.1.3执行一条select语句，然后查看硬解析次数

SYS@anqing2(rac2)> select * from ta where id=168;

ID NAME

---------- ------------------------------

168 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882061

parse time elapsed2648196

parse count (total)12780360

parse count (hard)9890021

parse count (failures)71

--这里硬解析的次数加一，因为之前SQL没有解析过，所以需要进行硬解析之后才能执行。

2.1.4执行与之前类似的SQL，谓词值不一样

SYS@anqing2(rac2)> select * from ta where id=198;

ID NAME

---------- ------------------------------

198 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882061

parse time elapsed2648196

parse count (total)12780482

parse count (hard)9890022

parse count (failures)71

--硬解析次数又加1了，没有重用之前的执行计划

2.1.5执行相同的SQL语句

SYS@anqing2(rac2)> select * from ta where id=198;

ID NAME

---------- ------------------------------

198 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882061

parse time elapsed2648196

parse count (total)12780543

parse count (hard)9890022

parse count (failures)71

--测试硬解析没有变化。重用之前的cursor。

总结：

在这种模式下,只有SQL语句完全相同的情况下，才会使用相同的cursor，即执行计划。

这种模式下，表有统计信息和没有统计信息的执行计划是有出入的。所以该模式下的表，需要定期的去收集统计信息。

2.2 cursor_sharing=force

--修改cursor_sharing值

SYS@anqing2(rac2)>alter session set cursor_sharing='force';

Session altered.

SYS@anqing2(rac2)>show parameter cursor_sharing

NAMETYPEVALUE

------------------------- --------------------- ---------------

cursor_sharingstringforce

--查看硬解析次数

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882075

parse time elapsed2648219

parse count (total)12782090

parse count (hard)9890067（硬解析次数）

parse count (failures)71

-- select查询

SYS@anqing2(rac2)>select * from ta where id=88;

ID NAME

---------- ------------------------------

88 dave

SYS@anqing2(rac2)>select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882075

parse time elapsed2648219

parse count (total)12782215

parse count (hard)9890068 --硬解析次数加一

parse count (failures)71

--执行相同的select，但谓词值不一样

SYS@anqing2(rac2)>select * from ta where id=99;

ID NAME

---------- ------------------------------

99 dave

SYS@anqing2(rac2)>select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882075

parse time elapsed2648219

parse count (total)12782285

parse count (hard)9890068

parse count (failures)71

--注意，这里的硬解析次数没有变化，这个就是force的作用。只要sql语句相同，不管谓词值是否相同，都会当成相同的sql，重用之前的cursor，不会进行硬解析。

--查看child cursor信息

SYS@anqing2(rac2)>select sql_text,child_number from v$sql where sql_text like 'select * from ta where%';

SQL_TEXTCHILD_NUMBER

---------------------------------------- ------------

select * from ta where id=:"SYS_B_0"0

select * from ta where id=:"SYS_B_0"1

select * from ta where id=:"SYS_B_0"2

注意：

对于相同的SQL，oracle在这里将不同的谓词值改成了变量，这样SQL_TEXT就相同，正常情况下，应该使用同一个cursor，即执行计划，但是在我上面的查询中，Oracle并没有重用，而是重新生成了一个child_cursor.这就说明Oracle认为这个cursor并不是最优的，所有重新生成了一个。

可以通过如下SQL查看为什么没有重用之前的cursor：

SQL>select * fromv$sql_shared_cursorwhere sql_id='c9swtz4spq3xz';

如果这里有Y，就是导致不能重用的原因。

Oracle性能相关的几个视图和参数

http://blog.csdn.net/xujinyang/article/details/6831385

总结：

Allows the creation of a new cursor if sharing an existing cursor, or if the cursor plan is not optimal.

When CURSOR_SHARING is set to FORCE,the database uses one parent cursor and one child cursor for each distinct SQL statement.The database uses the same plan for each execution of the same statement.

FORCE limits the growth of child cursors that can occur when the setting is SIMILAR.

当cursor_sharing设置为force时，Oracle会把相同SQL的不同谓词值转换成变量，这样SQL_TEXT就看上去一样。Oracle就会使用一个相同的cursor。这样他们的执行计划也是一样的。

当Oracle认为存在的cursor不是最优的时候，就会重新创建一个child cursor，而不重用之前的已经存在cursor。可以通过v$sql_shared_cursor查看为什么没有重用。

这样就会和我们上面查询的一样，会有多个child cursor，但是他们的parent cursor是一样的。这个child cursor不是无限增常的，force和similar都会限制child cursor的增长。

2.3 cursor_sharing=similar

在这种模式下，对表做统计和没做统计分析是不一样的。有关统计信息，参考我的Blog：

Oracle Statistic统计信息小结

http://blog.csdn.net/xujinyang/article/details/6881672

2.3.1没有统计信息，没有直方图的情况

--查询ta的统计信息

SYS@anqing2(rac2)> exec dbms_stats.delete_table_stats('SYS','TA');

PL/SQL procedure successfully completed.

--修改cursor_sharing模式

SYS@anqing2(rac2)>alter session set cursor_sharing='similar';

Session altered.

SYS@anqing2(rac2)>show parameter cursor_sharing

NAMETYPEVALUE

---------------------- ------------------ ----------------

cursor_sharingstringsimilar

在similer模式下，且没有直方图，等于force模式。看测试：

--查看硬解析次数

SYS@anqing2(rac2)>select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882222

parse time elapsed2648406

parse count (total)12793472

parse count (hard)9890538

parse count (failures)74

--做一次select查询

SYS@anqing2(rac2)> select * from ta where id=238;

ID NAME

---------- ------------------------------

238 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882222

parse time elapsed2648406

parse count (total)12793476

parse count (hard)9890539硬解析加一

parse count (failures)74

--在次select

SYS@anqing2(rac2)>select * from ta where id=2397;

ID NAME

---------- ----------

2397 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse time cpu1882337

parse time elapsed2648558

parse count (total)12823605

parse count (hard)9890751

parse count (failures)75

--这里还是有增加硬解析，有点奇怪

--查看对应的SQL

SYS@anqing2(rac2)> select sql_text,child_number from v$sql where sql_text like 'select * from ta where%';

SQL_TEXTCHILD_NUMBER

---------------------------------------- ------------

select * from ta whereid=2380

select * from ta whereid=23960

select * from ta where id=23910

select * from ta where id=23970

这里并没有将谓词值转换成变量，也就是说similar没有生效，没有达到force的效果。

在如下Blog上有对Force没有生效的一个讨论：

http://forums.oracle.com/forums/message.jspa?messageID=3569923#3569923

摘取一段如下：

Interesting. You may need to flush the shared pool "harder"... Try your testcase again, but issue theALTER SYSTEM FLUSH SHARED_POOL twice in a row before changing the cursor_sharing setting.It looks like that the statement doesn't show up in V$SQLAREA any longer but still seems not to be flushed entirely and I can find some left-overs of it in the underlying X$KGL library cache fixed tables with the same hash value.After the second "flush shared pool" these are gone and then the cursor sharing works as expected.

解决方法就是在指定cursor­_sharing之前，执行2次flush shared_pool.测试一下看看。

SYS@anqing2(rac2)> alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)> alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)> alter session set cursor_sharing=similar;

Session altered.

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse count (hard)9890936

SYS@anqing2(rac2)>select * from ta where id=232;

ID NAME

---------- ----------

232 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse count (hard)9890946

--从硬解析来看，数量有增加

--在查询一次

SYS@anqing2(rac2)> select * from ta where id=23218;

ID NAME

---------- ----------

23218 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAMEVALUE

------------------------------ ----------

parse count (hard)9890954

--硬解析又增加了，如果说只有这一条SQL执行，不会增加硬解析。这个是其他的SQL带来的硬解析，我们可以查看v$sql验证：

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select * from ta where%';

SQL_TEXTCHILD_NUMBEREXECUTIONS

---------------------------------------- ------------ ----------

select * from ta where id=:"SYS_B_0"02

--在这里，已经使转成成了变量，并且执行了2次。这说明，我们的force是生效的。

2.3.2有统计信息和直方图

当cursor_sharing设置为similar，并且表上存在histograms时，这时候的similar等于默认值exact.下面我们验证一下。

--分析表

SYS@anqing2(rac2)> exec dbms_stats.gather_table_stats('SYS','TA',METHOD_OPT =>'FOR COLUMNS SIZE 10 ID');

PL/SQL procedure successfully completed.

--这里我手工的指定了收集的类型。

--查看直方图: histograms

这部分内容，具体可以参考我的blog：Oracle Statistic统计信息小结

http://blog.csdn.net/xujinyang/article/details/6881672

SYS@anqing2(rac2)> SELECT column_name, num_distinct, num_buckets, histogram

2FROM DBA_TAB_COL_STATISTICS WHERE table_name = 'TA' AND column_name = 'ID';

COLUMN_NAMENUM_DISTINCT NUM_BUCKETS HISTOGRAM

------------------------------ ------------ ----------- ---------------

ID100551010HEIGHT BALANCED

--先flush shared_pool两次

SYS@anqing2(rac2)>alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)>alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)>show parameter cursor_sharing

NAMETYPEVALUE

------------------------ -------------------------------- ----------

cursor_sharingstringSIMILAR

-- select查询和对应的解析

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXTCHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"03

SYS@anqing2(rac2)>select count(*) from ta where id>10000;

COUNT(*)

----------

990000

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXTCHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"03

select count(*) from ta where id>:"SYS_B_0"11

--因为ID值和之前的不一样，所以没有重用之前的cursor，而是重新创建了一个

SYS@anqing2(rac2)>select count(*) from ta where id>100;

COUNT(*)

----------

999900

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXTCHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"03

select count(*) from ta where id>:"SYS_B_0"11

select count(*) from ta where id>:"SYS_B_0"21

--这次ID值又不一样，又重新生成了一个cursor

.....中间又生成了N次...

SYS@anqing2(rac2)>select count(*) from ta where id>90808;

COUNT(*)

----------

909192

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXTCHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"03

select count(*) from ta where id>:"SYS_B_0"11

select count(*) from ta where id>:"SYS_B_0"21

select count(*) from ta where id>:"SYS_B_0"31

select count(*) from ta where id>:"SYS_B_0"41

select count(*) from ta where id>:"SYS_B_0"51

select count(*) from ta where id>:"SYS_B_0"61

select count(*) from ta where id>:"SYS_B_0"71

--这个就像我们开始说的，虽然会对变量进行转换，但不会重用，只有谓词值一样的时候才重用之前的cursor。

SYS@anqing2(rac2)> select count(*) from ta where id>90808;

COUNT(*)

----------

909192

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXTCHILD_NUMBEREXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"03

select count(*) from ta where id>:"SYS_B_0"11

select count(*) from ta where id>:"SYS_B_0"21

select count(*) from ta where id>:"SYS_B_0"31

select count(*) from ta where id>:"SYS_B_0"41

select count(*) from ta where id>:"SYS_B_0"51

select count(*) from ta where id>:"SYS_B_0"61

select count(*) from ta where id>:"SYS_B_0"72

--注意这里，因为我们执行了一个一样的SQL,包括谓词，这里的重用了之前的cursor。

总结：

当cursor_sharing设置为similar时，Oracle会对SQL的位置值转换成常量，这个和force一样。但是不同的是：

（1）当字段上有histograms时，这时等于exact，虽然SQL text一样，但谓词值不同就不会重用cursor。只有SQL语句完全一样，包括谓词值，才会重用cursor。

（2）当没有statstics时，即没有histograms，这时等于force，只要SQL语句一样，就会重用cursor。

三.similar模式对expdp带来的影响

在eygle的blog上看到similar对增加expdp备份的时间。

原理链接如下：

http://www.eygle.com/archives/2011/01/cursor_sharing_expdp.html

相关链接：

Oracle 10g Data Pump Expdp/Impdp详解

http://blog.csdn.net/xujinyang/article/details/6830446

Oracle expdp/impdp使用示例

http://blog.csdn.net/xujinyang/article/details/6831273

3.1按照默认的expdp

SYS@anqing2(rac2)> show parameter cursor_sharing

NAMETYPEVALUE

------------------------------------ ----------- ------------------------------

cursor_sharingstringEXACT

[oracle@rac2 backup]$expdp /'/ as sysdba/' directory=backup full=y dumpfile=fullexp.dmp logfile=fullexp.log

Export: Release 10.2.0.4.0 - Production on Friday, 17 June,2011 13:54:40

...............

Job "SYS"."SYS_EXPORT_FULL_01" successfully completed at 14:05:49

测试库，全备花了11分钟。

3.2用similar模式expdp备份

cursor_sharing是一个动态参数，可以在执行expdp之前进行修改，然后执行导出,备份完在改回来。

SYS@anqing2(rac2)>alter system set cursor_sharing=similar scope=memory;

System altered.

SYS@anqing2(rac2)> show parameter cursor_sharing

NAMETYPEVALUE

------------------------------------ ----------- ------------------------------

cursor_sharingstringSIMILAR

[oracle@rac2 backup]$ expdp /'/ as sysdba/' directory=backup full=y dumpfile=fullexp.dmp logfile=fullexp.log

Export: Release 10.2.0.4.0 - Production on Friday, 17 June,2011 13:40:37

....

Job "SYS"."SYS_EXPORT_FULL_01" successfully completed at13:52:42

备份了12分钟.

因为库不大，所以差距不明显.不过如果库大的话，一般也不会用expdp来备份。所以这里作为一个知识点，了解一下。