经常查看的一些命中率

命中率

1)Library Cache的命中率:

.计算公式:Library Cache Hit Ratio = sum(pinhits) / sum(pins)

SELECT SUM(pinhits)/sum(pins) FROM V$LIBRARYCACHE;

通常在98%以上，否则，需要要考虑加大共享池，绑定变量，修改cursor_sharing等参数。

2)计算共享池内存使用率:

SELECT (1 - ROUND(BYTES / (&TSP_IN_M * 1024 * 1024), 2)) * 100 || '%'
FROM V$SGASTAT
WHERE NAME = 'free memory' AND POOL = 'shared pool';

其中: &TSP_IN_M是你的总的共享池的SIZE(M)

共享池内存使用率，应该稳定在75%-90%间，太小浪费内存，太大则内存不足。

 

查询空闲的共享池内存:

SELECT pool,name,BYTES/1024/1024 "MB" FROM V$SGASTAT
WHERE NAME = 'free memory' AND POOL = 'shared pool';

3)db buffer cache命中率:

计算公式:Hit ratio = 1 - [physical reads/(block gets + consistent gets)]

SELECT NAME, PHYSICAL_READS, DB_BLOCK_GETS, CONSISTENT_GETS,
1 - (PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS)) "Hit Ratio"
FROM V$BUFFER_POOL_STATISTICS
WHERE NAME='DEFAULT';

通常应在90%以上，否则，需要调整,加大DB_CACHE_SIZE

---第二种办法：oracle官方文档方法

命中率的计算公式为:
 Hit Ratio = 1 - ((physical reads - physical reads direct - physical reads direct (lob)) /
(db block gets + consistent gets - physical reads direct - physical reads direct (lob))

分别代入上一查询中的结果值,就得出了Buffer cache的命中率

SELECT NAME, VALUE
FROM V$SYSSTAT
WHERE NAME IN('session logical reads',
'physical reads',
'physical reads direct',
'physical reads direct (lob)',
'db block gets', 'consistent gets');

4)数据缓冲区命中率：

select value from v$sysstat where name ='physical reads';
select value from v$sysstat where name ='physical reads direct';
select value from v$sysstat where name ='physical reads direct (lob)';
select value from v$sysstat where name ='consistent gets';
select value from v$sysstat where name = 'db block gets';

这里命中率的计算应该是
令 x = physical reads direct + physical reads direct (lob)
命中率 =100 - ( physical reads - x) / (consistent gets + db block gets - x)*100
通常如果发现命中率低于90%,则应该调整应用可可以考虑是否增大数据缓冲区

5)共享池的命中率：

select sum(pinhits-reloads)/sum(pins)*100 "hit radio" from v$librarycache;

假如共享池的命中率低于95%,就要考虑调整应用（通常是没使用bind var ）或者增加内存

 

6)计算在内存中排序的比率:

SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';―查询内存排序数
SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';―查询磁盘排序数

--caculate sort in memory ratio
SELECT round(&sort_in_memory/(&sort_in_memory+&sort_in_disk),4)*100||'%' FROM dual;

此比率越大越好,太小整要考虑调整,加大PGA

7)PGA的命中率:

计算公式:BP x 100 / (BP + EBP)

BP: bytes processed

EBP: extra bytes read/written

SELECT * FROM V$PGASTAT  WHERE NAME='cache hit percentage';

或者从OEM的图形界面中查看

我们可以查看一个视图以获取Oracle的建议值:

SELECT round(PGA_TARGET_FOR_ESTIMATE/1024/1024) target_mb,
ESTD_PGA_CACHE_HIT_PERCENTAGE cache_hit_perc,
ESTD_OVERALLOC_COUNT
FROM V$PGA_TARGET_ADVICE;

TARGET_MB  CACHE_HIT_PERC  ESTD_OVERALLOC_COUNT

----------           --------------        --------------------

   63              23                   367

  125              24                    30

  250              30                     3

  375              39                     0

  500              58                     0

  600              59                     0

  700              59                     0

  800              60                     0

  900              60                     0

在此例中:PGA至少要分配375M

我个人认为PGA命中率不应该低于50%

以下的SQL统计sql语句执行在三种模式的次数: optimal memory size, one-pass memory size, multi-pass memory size:

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

8)共享区字典缓存区命中率

计算公式：SUM(gets - getmisses - usage -fixed) / SUM(gets)

命中率应大于0.85

select sum(gets-getmisses-usage-fixed)/sum(gets)
from v$rowcache;

9)数据高速缓存区命中率

计算公式：1-(physical reads / (db block gets + consistent gets))

命中率应大于0.90最好

select name,value
from v$sysstat
where name in ('physical reads','db block gets','consistent gets');

10)共享区库缓存区命中率

计算公式：SUM(pins - reloads) / SUM(pins)

命中率应大于0.99

select sum(pins-reloads)/sum(pins)
from v$librarycache;

11)检测回滚段的争用

SUM(waits)值应小于SUM(gets)值的1%

select sum(gets),sum(waits),sum(waits)/sum(gets)
from v$rollstat;

　　

12)检测回滚段收缩次数

select name,shrinks
from v$rollstat, v$rollname
where v$rollstat.usn = v$rollname.usn;