The usage of V$PGA_TARGET_ADVICE

Oracle 10g 给出了一系列的自动优化的建议，告诉我们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：
SQL> SELECT pga_target_for_estimate / 1024 / 1024 "PGA(MB)",pga_target_factor,estd_pga_cache_hit_percentage,estd_overalloc_count FROM v$pga_target_advice;

PGA(MB) PGA_TARGET_FACTOR ESTD_PGA_CACHE_HIT_PERCENTAGE ESTD_OVERALLOC_COUNT
---------- ----------------- ----------------------------- --------------------
        10              .125                           100                    1
        20               .25                           100                    1
        40                .5                           100                    1
        60               .75                           100                    0
        80                 1                           100                    0
        96               1.2                           100                    0
       112               1.4                           100                    0
       128               1.6                           100                    0
       144               1.8                           100                    0
       160                 2                           100                    0
       240                 3                           100                    0

PGA(MB) PGA_TARGET_FACTOR ESTD_PGA_CACHE_HIT_PERCENTAGE ESTD_OVERALLOC_COUNT
---------- ----------------- ----------------------------- --------------------
       320                 4                           100                    0
       480                 6                           100                    0
       640                 8                           100                    0

14 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80M，

通过pga_aggregate_target参数可以确认一下
SQL> show parameter pga_aggregate_target

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- ------------------------------
pga_aggregate_target                 big integer                      80M

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10M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60M的时候，可以消除PGA的过载。

5.通过上面的结论，我们可以将PGA的大小设置为60M。
SQL> alter system set pga_aggregate_target=60m;

System altered.

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

PGA(MB) PGA_TARGET_FACTOR ESTD_PGA_CACHE_HIT_PERCENTAGE ESTD_OVERALLOC_COUNT
---------- ----------------- ----------------------------- --------------------
        18                .5                            94                    2
        27               .75                            94                    2
        60                 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.

通过上面的描述，您一定已经会了解了这个v$pga_target_advice视图了。