命中率指标

命中率指标虽然非绝对，但也是个重要的参考依据

1.数据缓冲区的使用命中率：
SQL> select name, value from v$sysstat where name in ('db block gets', 'consistent gets', 'physical reads');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
db block gets                                                        574110
consistent gets                                                     1191845
physical reads                                                        18215



来查看数据库数据缓冲区的使用情况。
查询出来的结果可以计算出来数据缓冲区的使用命中率＝
1 - (physical reads / (db block gets + consistent gets))。 
  
  这个命中率应该在90％以上，否则需要增加数据缓冲区的大小。


2.buffer cache命中率
select 1-((physical.value - direct.value - lobs.value)/logical.value) "Buffer Cache Hit Ratio"
   from V$SYSSTAT physical, V$SYSSTAT direct, V$SYSSTAT lobs, V$SYSSTAT logical
    where physical.name = 'physical reads'
    And direct.name = 'physical reads direct' AND lobs.name = 'physical reads direct (lob)'
　　And logical.name = 'session logical reads';



SQL>
 
Buffer Cache Hit Ratio
----------------------
     0.989616782275326