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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