欢迎进入Oracle社区论坛,与200万技术人员互动交流 >>进入 在Oracle9i之前,PGA的计算和控制都是比较复杂的事情,从Oracle9i开始,Oracle提供了一种SQL内存管理的新方法:自动化SQL执行内存管理(Automated SQL Execution Memory Management),使用这个新特
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在Oracle9i之前,PGA的计算和控制都是比较复杂的事情,从Oracle9i开始,Oracle提供了一种SQL内存管理的新方法:自动化SQL执行内存管理(Automated SQL Execution Memory Management),使用这个新特性,Oracle可以自动调整S Q L内存区,而不用关闭数据库,这一改进大大简化了DBA的工作,同时也提高了Oracle数据库的性能。
为实现自动的PGA管理,Oracle引入了几个新的初始化参数:
1.PGA_AGGREGATE_TARGET-此参数用来指定所有session总计可以使用最大PGA内存。这个参数可以被动态的更改,取值范围从10M ―― (4096G-1 )bytes. 2.WORKAREA_SIZE_POLICY-此参数用于开关PGA内存自动管理功能,该参数有两个选项:AUTO 和 MANUAL,当设置为AUTO时,数据库使用Oracle9i提供的自动PGA管理功能,当设置为MANUAL时,则仍然使用Oracle9i前手工管理的方式。
缺省的,Oracle9i中WORKAREA_SIZE_POLICY被设置为AUTO.
需要注意的是,在Oracle9i中,PGA_AGGREGATE_TARGET参数仅对专用服务器模式下(Dedicated Server)的专属连接有效,但是对共享服务器(Shared Server)连接无效;从Oracle10g开始PGA_AGGREGATE_TARGET对专用服务器连接和共享服务器连接同时生效。
PGA_AGGREGATE_TARGET 参数同时限制全局PGA分配和私有工作区内存分配:
1.对于串行操作,单个SQL操作能够使用的PGA内存按照以下原则分配:MIN(5% PGA_AGGREGATE_TARGET,100MB)
2.对于并行操作30% PGA_AGGREGATE_TARGET /DOP (DOP=Degree Of Parallelism 并行度)
要理解PGA的自动调整,还需要区分可调整内存(TUNABLE MEMORY SIZE)与不可调整内存(UNTUNABLE MEMORY SIZE)。可调整内存是由SQL工作区使用的,其余部分是不可调整内存。
启用了自动PGA调整之后, Oracle仍然需要遵循以下原则:
UNTUNABLE MEMORY SIZE + TUNABLE MEMORY SIZE
数据库系统只能控制可调整部分的内存分配,如果可调整的部分过小,则Oracle永远也不会强制启用这个等式。
另外,PGA_AGGREGATE_TARGET参数在CBO优化器模式下,对于SQL的执行计划会产生影响。Oracle在评估执行计划时会根据PGA_AGGREGATE_TARGET参数评估在Sort,HASH-JOIN或Bitmap操作时能够使用的最大或最小内存,从而选择最优的执行计划。
对于PGA_AGGREGATE_TARGET参数的设置,Oracle提供这样一个建议方案1.对于OLTP系统PGA_AGGREGATE_TARGET = (
也就是说,对于一个单纯的数据库服务器,通常我们需要保留20%的物理内存给操作系统使用,剩余80%可以分配给Oracle使用。Oracle使用的内存分为两部分SGA和PGA,那么PGA可以占用Oracle消耗总内存的20%(OLTP系统)至50%(DSS系统)。
这只是一个建议设置,更进一步的我们应该根据数据库的具体性能指标来调整和优化PGA的使用。

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