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关于Oracle中各个命中率的计算以及相关的调优
1、Library Cache的命中率
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
3、空闲的共享池内存
SELECT * FROM V$SGASTAT WHERE NAME = 'free memory' AND POOL = 'shared pool';4、db buffer cache命中率
#计算公式：Hit ratio = 1 - [physical reads/(block gets + consistent gets)]
SELECT NAME, PHYSICAL_READS, DB_BLOCK_GETS, CONSISTENT_GETS,
1 - (PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS)) "Hit Ratio"
FROM V$BUFFER_POOL_STATISTICS
WHERE NAME='DEFAULT';通常应在90%以上，否则，需要调整,加大DB_CACHE_SIZE
5、数据缓冲区命中率
SQL> select value from v$sysstat where name ='physical reads';
SQL> select value from v$sysstat where name ='physical reads direct';
SQL> select value from v$sysstat where name ='physical reads direct (lob)';
SQL> select value from v$sysstat where name ='consistent gets';
SQL> select value from v$sysstat where name = 'db block gets';这里命中率的计算应该是
令 x = physical reads direct + physical reads direct (lob)
命中率 =100 – ( physical reads – x) / (consistent gets + db block gets – x)*100
通常如果发现命中率低于90%,则应该调整应用可可以考虑是否增大数据缓冲区
6、共享池的命中率（重要）
select sum(pinhits-reloads)/sum(pins)*100 "hit radio" from v$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
7、计算在内存中排序的比率
SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';—查询内存排序数
SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';—查询磁盘排序数
SQL>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
8、PGA的命中率
#计算公式:BP x 100 / (BP + EBP)
BP: bytes processed
EBP: extra bytes read/written
SQL>SELECT * FROM V$PGASTAT WHERE NAME='cache hit percentage';或者从OEM的图形界面中查看
我们可以查看一个视图以获取Oracle的建议值：
SQL>SELECT round(PGA_TARGET_FOR_ESTIMATE/1024/1024) target_mb,
ESTD_PGA_CACHE_HIT_PERCENTAGE cache_hit_perc,
ESTD_OVERALLOC_COUNT
FROM V$PGA_TARGET_ADVICE;
The output of this query might look like the following:
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%
9、共享区字典缓存区命中率
#计算公式：SUM(gets - getmisses - usage -fixed) / SUM(gets)
SQL>select sum(gets-getmisses-usage-fixed)/sum(gets)
from v$rowcache;命中率应大于0.85
10、数据高速缓存区命中率
#计算公式：1-(physical reads / (db block gets + consistent gets))
SQL>select name,value
from v$sysstat
where name in ('physical reads','db block gets','consistent gets');命中率应大于0.9最好
11、共享区库缓存区命中率
#计算公式：SUM(pins - reloads) / SUM(pins)
SQL>select sum(pins-reloads)/sum(pins)
from v$librarycache;命中率应大于0.99
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