Oracle数据库提高命中率及相关优化(转)
本文是关于Oracle数据库调试与优化方面的文章，主要介绍Oracle数据库中命中率相关的问题，包括不同的算法之间性能的比对。51CTO编辑向您推荐《Oracle数据库调试与性能优化》

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:

.计算公式:Library Cache Hit Ratio = sum(pinhits) / sum(pins)
SQL>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:
SQL>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:
SQL>SELECT * 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 -
SQL>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

另外一种计算命中率的方法(摘自ORACLE官方文档<<数据库性能优化>>):

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
SQL>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：
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

5)共享池的命中率：
SQL> 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:
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 
--caculate sort in memory ratio 
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

7)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            &nbs