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HomeDatabaseMysql TutorialOracle 的 cursor_sharing参数

这个参数的设置,oracle是为了满足一些以前开发的程序,里面有大量的similar statement,但是重写又不现实的情况下使用的一个参数

先来看看官方文档中对这个参数的解释

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


CURSOR_SHARINGdetermines what kind of SQL statements can share the same cursors.

Values:

FORCE

Forces statements that may differ in some literals, but are otherwise identical, to share a cursor, unless the literals affect the meaning of the statement.

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.

EXACT

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

参数cursor_sharing的解释

这个参数的设置,Oracle是为了满足一些以前开发的程序,里面有大量的similar statement,但是重写又不现实的情况下使用的一个参数。(并且oracle也不建议使用这个参数)

什么时候需要修改这个参数呢?需要满足以下的条件。

一个是由于大量的shared pool hit miss影响了用户的响应时间(就是当前的shared pool无法满足共享sql语句存储的需要,Alan:当前libary cache中没有我们所需要重用的explain和sql),如果没有这个问题,那么设置这个参数,,可能会造成更糟糕的性能。这个参数只会减少parse的时间。

另外一个就是在现有程序中有大量的similar statement,可以通过设置这个参数来获得比较好的性能。

cursor_sharing这个参数有三个值可选,exact、similar、force。当值为exact时为默认值,也是oracle的默认处理方式。就是当一个statement parse的时候,首先到shared pool区查看是否有exact statement存在(就是看是否在shared pool中有和当前要解析的statement完全一样的语句存在),如果不存在,就执行hard parse

如果该参数设置为similar,那么如果在shared pool中无法找到exact statement的存在的时候,就会在shared pool进行一次新的查找,就是查找和当前要解析的语句是否是similar statement的语句。这里需要对similar statement进行解释,similar statement就是除了value of some literal不同的语句,别的地方都相同的语句。比如下面:

select * from a where a=1;

select * from a where a=2;

当cursor_sharing设置为similar时,如果在shared pool中查找到这样的语句,就会做下一步的检查,看shared pool中缓存的这个语句的execution plan是否适合当前解析的语句,如果适合,就会使用shared pool的语句,而不去做hard parse。如果cursor_sharing设置为force的时候,当在shared pool中发现了similar statement之后,就不会再去检查执行计划了,而直接使用在shared pool的这个语句了。

将cursor_sharing设置为force实际上是危险的。这会可能形成sub optimal的执行计划。比如对于一个范围查找的语句,比如

select * from a where a>10 and a

这样看起来是减少了parse恶的时间,但是大大增大了execution的时间。

对于新开发的application,最好是不要设置这个参数,而是针对可以共享的语句使用绑定变量,而对于不适合共享的语句,就不使用绑定变量。将cursor_sharing保持默认值,也就是exact。

调整cursor_sharing的关键一定是要看调整cursor_sharing的条件是否存在。

就是是否有比较大的shared pool hit miss,如果这个条件没有,那就没有必要调整这个参数。cursor_sharing的目的是减少parse time,但是在整个db time中,可能parse time只占很小的一部分。

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