通过10g新特性得到PGA的调整建议【转自http://space.itpub.net/519536 】

10g Oracle给出了一系列的自动优化的建议，PGA分配多大能给系统带来最大的性能？V$PGA_TARGET_ADVICE视图给出了很好的“预测”！

看一下这个视图能给我们带来什么样的信息（视图中每个列都很有帮助）：
sys@ora10g> SELECT   pga_target_for_estimate / 1024 / 1024 "PGA(MB)",
  2           pga_target_factor,
  3           estd_pga_cache_hit_percentage,
  4           estd_overalloc_count
  5    FROM   v$pga_target_advice;

   PGA(MB) PGA_TARGET_FACTOR ESTD_PGA_CACHE_HIT_PERCENTAGE ESTD_OVERALLOC_COUNT
---------- ----------------- ----------------------------- --------------------
        10                .5                            34                   13
        15               .75                            34                   13
        20                 1                           100                   13
        24               1.2                           100                   13
        28               1.4                           100                   13
        32               1.6                           100                    3
        36               1.8                           100                    0
        40                 2                           100                    0
        60                 3                           100                    0
        80                 4                           100                    0
       120                 6                           100                    0
       160                 8                           100                    0

12 rows selected.

通过上面的数据可以得到如下的结论：
1.第一列表示不同的PGA的具体值

2.第二列PGA_TARGET_FACTOR为“1”表示当前的pga_aggregate_target设置大小（其他数值都是以这个数据为基础的倍数），我这里是20M，通过pga_aggregate_target参数可以确认一下
sys@ora10g> show parameter pga_aggregate_target;

NAME                    TYPE                 VALUE
----------------------- -------------------- -----------
pga_aggregate_target    big integer          20M

3.第三列表示PGA的估算得到的Cache命中率的百分比
目前系统如果PGA为20M的时候，就可以达到100%的命中率

4.第四列如果为“0”表示可以消除PGA的过载
从上面的数据中可以得到，当PGA为36M的时候，可以消除PGA的过载。

5.综合以上的结论，我们最终可以将PGA的大小设置为36M。
sys@ora10g> alter system set pga_aggregate_target=36m;

System altered.

6.调整后，再次查询一下v$pga_target_advice视图得到如下的建议信息，可以看到基本上已经满足现在的系统需求。
sec@ora10g> SELECT pga_target_for_estimate / 1024 / 1024 "PGA(MB)",
  2         pga_target_factor,
  3         estd_pga_cache_hit_percentage,
  4         estd_overalloc_count
  5  FROM   v$pga_target_advice;

   PGA(MB) PGA_TARGET_FACTOR ESTD_PGA_CACHE_HIT_PERCENTAGE ESTD_OVERALLOC_COUNT
---------- ----------------- ----------------------------- --------------------
        18                .5                            94                    2
        27               .75                            94                    2
        36                 1                           100                    0
43.1992188               1.2                           100                    0
50.3994141               1.4                           100                    0
57.5996094               1.6                           100                    0
64.7998047               1.8                           100                    0
        72                 2                           100                    0
       108                 3                           100                    0
       144                 4                           100                    0
       216                 6                           100                    0
       288                 8                           100                    0

12 rows selected.

恭喜你，到这里，您一定已经会“看”这个v$pga_target_advice视图了。

这是一个动态的过程，可以定期的查看这个视图得到更加有效的PGA大小的设置建议。

【OEM方法】同样是上面的建议信息，我们可以通过Oracle的OEM得到更加直观的信息。
1.使用自己的服务器的IP地址登陆到OEM界面
http://144.194.192.183:1158/em/console/logon/logon

2.数据用户名（sys）和密码（sys的密码），“Connect As”选择“SYSDB”，最后点击“Login”

3.最上面有三个大的可选菜单“Home”、“Performance”、“Administration”和“Maintenance”，需要选择“Administration”，在“Database Configuration”大类中的第一个就是“Memory Parameters”，点击进入，此时您会看到“SGA”和“PGA”，点击第二个“PGA”，OK，这里就可以看到“Advice”和“PGA Memory Usage Details”两个按钮，第一个“Advice”按钮得到的信息就是我们上面通过v$pga_target_advice视图得到的信息，这里会更加直观的，以曲线图的形式进行展示。
另外一个按钮“PGA Memory Usage Details”是通过视图v$pga_target_advice_histogram得到的一个统计柱状图。

附图如下：








【附视图官方解释】10g官方文档中关于v$pga_target_advice和v$pga_target_advice_histogram两个视图的说明

V$PGA_TARGET_ADVICE

V$PGA_TARGET_ADVICEpredicts how the cache hit percentage and over allocation count statistics displayed by theV$PGASTATperformance view would be impacted if the value of thePGA_AGGREGATE_TARGETparameter is changed. The prediction is performed for various values of thePGA_AGGREGATE_TARGETparameter, selected around its current value. The advice statistic is generated by simulating the past workload run by the instance.

The content of the view is empty ifPGA_AGGREGATE_TARGETis not set. In addition, the content of this view is not updated if theSTATISTICS_LEVELparameter is set toBASIC. Base statistics for this view are reset at instance startup and when the value of thePGA_AGGREGATE_TARGETinitialization parameter is dynamically modified.

Column	Datatype	Description
PGA_TARGET_FOR_ESTIMATE	NUMBER	Value ofPGA_AGGREGATE_TARGETfor this prediction (in bytes)
PGA_TARGET_FACTOR	NUMBER	PGA_TARGET_FOR_ESTIMATE/ the current value of thePGA_AGGREGATE_TARGETparameter
ADVICE_STATUS	VARCHAR2(3)	Indicates whether the advice is enabled (ON) or disabled (OFF) depending on the value of theSTATISTICS_LEVELparameter
BYTES_PROCESSED	NUMBER	Total bytes processed by all the work areas considered by this advice (in bytes)
ESTD_EXTRA_BYTES_RW	NUMBER	Estimated number of extra bytes which would be read or written ifPGA_AGGREGATE_TARGETwas set to the value of thePGA_TARGET_FOR_ESTIMATEcolumn. This number is derived from the estimated number and size of work areas which would run in one-pass (or multi-pass) for that value ofPGA_AGGREGATE_TARGET.
ESTD_PGA_CACHE_HIT_PERCENTAGE	NUMBER	Estimated value of the cache hit percentage statistic whenPGA_AGGREGATE_TARGETequalsPGA_TARGET_FOR_ESTIMATE. This column is derived from the above two columns and is equal toBYTES_PROCESSED/ (BYTES_PROCESSED+ESTD_EXTRA_BYTES_RW)
ESTD_OVERALLOC_COUNT	NUMBER	Estimated number of PGA memory over-allocations if the value ofPGA_AGGREGATE_TARGETis set toPGA_TARGET_FOR_ESTIMATE. A nonzero value means thatPGA_TARGET_FOR_ESTIMATEis not large enough to run the work area workload. Hence, the DBA should not setPGA_AGGREGATE_TARGETtoPGA_TARGET_FOR_ESTIMATEsince Oracle will not be able to honor that target.

V$PGA_TARGET_ADVICE_HISTOGRAM

V$PGA_TARGET_ADVICE_HISTOGRAMpredicts how statistics displayed by theV$SQL_WORKAREA_HISTOGRAMdynamic view would be impacted if the value of thePGA_AGGREGATE_TARGETparameter is changed. This prediction is performed for various values of thePGA_AGGREGATE_TARGETparameter, selected around its current value. The advice statistic is generated by simulating the past workload run by the instance.

The content of the view is empty ifPGA_AGGREGATE_TARGETis not set. In addition, the content of this view is not updated when theSTATISTICS_LEVELinitialization parameter is set toBASIC. Base statistics for this view are reset at instance startup or when the value of thePGA_AGGREGATE_TARGETinitialization parameter is dynamically modified.

Column	Datatype	Description
PGA_TARGET_FOR_ESTIMATE	NUMBER	Value ofPGA_AGGREGATE_TARGETfor this prediction (in bytes)
PGA_TARGET_FACTOR	NUMBER	PGA_TARGET_FOR_ESTIMATE/ the current value of thePGA_AGGREGATE_TARGETparameter
ADVICE_STATUS	VARCHAR2(3)	Indicates whether the advice is enabled (ON) or disabled (OFF) depending on the value of theSTATISTICS_LEVELparameter
LOW_OPTIMAL_SIZE	NUMBER	Lower bound for the optimal memory requirement of work areas included in this row (in bytes)
HIGH_OPTIMAL_SIZE	NUMBER	Upper bound for the optimal memory requirement of work areas included in this row (in bytes)
ESTD_OPTIMAL_EXECUTIONS	NUMBER	Number of work areas with an optimal memory requirement comprised betweenLOW_OPTIMAL_SIZEandHIGH_OPTIMAL_SIZEwhich are predicted to run optimal given a value ofPGA_AGGREGATE_TARGETequal toPGA_TARGET_FOR_ESTIMATE
ESTD_ONEPASS_EXECUTIONS	NUMBER	Number of work areas with an optimal memory requirement comprised betweenLOW_OPTIMAL_SIZEandHIGH_OPTIMAL_SIZEwhich are predicted to run one-pass given a value ofPGA_AGGREGATE_TARGETequal toPGA_TARGET_FOR_ESTIMATE
ESTD_MULTIPASSES_EXECUTIONS	NUMBER	Number of work areas with an optimal memory requirement comprised betweenLOW_OPTIMAL_SIZEandHIGH_OPTIMAL_SIZEwhich are predicted to run multi-pass given a value ofPGA_AGGREGATE_TARGETequal toPGA_TARGET_FOR_ESTIMATE
ESTD_TOTAL_EXECUTIONS	NUMBER	Sum ofESTD_OPTIMAL_EXECUTIONS,ESTD_ONEPASS_EXECUTIONS, andESTD_MULTIPASSES_EXECUTIONS
IGNORED_WORKAREAS_COUNT	NUMBER	Number of work areas with optimal memory requirement betweenLOW_OPTIMAL_SIZEandHIGH_OPTIMAL_SIZEignored in the advice generation due to memory and CPU constraints

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