[bbk2193] 第34集 - Chapter 09-Optimizing Sore Perations(03)

PGA Target Advice Statistics

V$PGA_TARGET_ADVICE predicts how cache hit percentages shown in V$PGASTAT evolve.

STATISTICS_LEVEL must be set to at least TYPICAL.（提示:如果想使V$PGA_TARGET_ADVICE工作，必须将STATISTIC_LEVEL修改成至少TYPICAL级别）

SQL> select * from v$PGA_TARGET_ADVICE;

PGA_TARGET_FOR_ESTIMATE PGA_TARGET_FACTOR ADV BYTES_PROCESSED  ESTD_TIME ESTD_EXTRA_BYTES_RW ESTD_PGA_CACHE_HIT_PERCENTAGE ESTD_OVERALLOC_COUNT
----------------------- ----------------- --- --------------- ---------- ------------------- ----------------------------- --------------------
163577856              .125 ON       6551463936   25831789            72195072                            99                    6
327155712               .25 ON       6551463936   25550234                   0                           100                    0
654311424                .5 ON       6551463936   25550234                   0                           100                    0
981467136               .75 ON       6551463936   25550234                   0                           100                    0
1308622848                 1 ON       6551463936   25550234                   0                           100                    0
1570347008               1.2 ON       6551463936   25550234                   0                           100                    0
1832071168               1.4 ON       6551463936   25550234                   0                           100                    0
2093796352               1.6 ON       6551463936   25550234                   0                           100                    0
2355520512               1.8 ON       6551463936   25550234                   0                           100                    0
2617245696                 2 ON       6551463936   25550234                   0                           100                    0
3925868544                 3 ON       6551463936   25550234                   0                           100                    0

PGA_TARGET_FOR_ESTIMATE PGA_TARGET_FACTOR ADV BYTES_PROCESSED  ESTD_TIME ESTD_EXTRA_BYTES_RW ESTD_PGA_CACHE_HIT_PERCENTAGE ESTD_OVERALLOC_COUNT
----------------------- ----------------- --- --------------- ---------- ------------------- ----------------------------- --------------------
5234491392                 4 ON       6551463936   25550234                   0                           100                    0
7851737088                 6 ON       6551463936   25550234                   0                           100                    0
1.0469E+10                 8 ON       6551463936   25550234                   0                           100                    0

14 rows selected.

将上述SQL语句，进行优化一下,如下:

SELECT ROUND(pga_target_for_estimate/1024/1024) AS target_mb,
estd_pga_cache_hit_percentage AS cach_hit_percent,
estd_overalloc_count
FROM v$pga_target_advice
ORDER BY target_mb;

TARGET_MB CACH_HIT_PERCENT ESTD_OVERALLOC_COUNT
---------- ---------------- --------------------
156               99                    6
312              100                    0
624              100                    0
936              100                    0
1248              100                    0
1498              100                    0
1747              100                    0
1997              100                    0
2246              100                    0
2496              100                    0
3744              100                    0

TARGET_MB CACH_HIT_PERCENT ESTD_OVERALLOC_COUNT
---------- ---------------- --------------------
4992              100                    0
7488              100                    0
9984              100                    0

14 rows selected.

解读优化后的SQL语句:以第一条记录为例:假如将PGA_AGGRGATE_TARGET参数值设置为156MB的话，缓存命中率为99%，溢出6块.以下记录依次类推.

Understanding the view

Limitation of Base Statistics

How to Tune

v$sysstat and v$sesstat

SQL> SELECT name profile,cnt,decode(total,0,0,round(cnt*100/total)) percentage
2  FROM (SELECT name,value cnt,(sum(value) over()) total
3        FROM V$SYSSTAT
4        WHERE name like 'workarea exec%'
5       );

PROFILE                                                                 CNT PERCENTAGE
---------------------------------------------------------------- ---------- ----------
workarea executions - optimal                                        180593        100
workarea executions - onepass                                             4          0
workarea executions - multipass                                           0          0