本文是關(guān)于Oracle數(shù)據(jù)庫調(diào)試與優(yōu)化方面的文章，主要介紹Oracle數(shù)據(jù)庫中命中率相關(guān)的問題，包括不同的算法之間性能的比對。

　　關(guān)于Oracle中各個命中率的計算以及相關(guān)的調(diào)優(yōu)

　　1)Library Cache的命中率:

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

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

　　通常在98%以上，否則，需要要考慮加大共享池，綁定變量，修改cursor_sharing等參數(shù)。

　　2)計算共享池內(nèi)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是你的總的共享池的SIZE(M)

　　共享池內(nèi)存使用率，應(yīng)該穩(wěn)定在75%-90%間，太小浪費內(nèi)存，太大則內(nèi)存不足。

　　查詢空閑的共享池內(nèi)存:

　　SQL>SELECT * FROM V$SGASTAT WHERE NAME = 'free memory' AND POOL = 'shared pool';

　　3)db buffer cache命中率:

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

　　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';

　　通常應(yīng)在90%以上，否則，需要調(diào)整,加大DB_CACHE_SIZE

　　另外一種計算命中率的方法(摘自O(shè)RACLE官方文檔<<數(shù)據(jù)庫性能優(yōu)化>>):

　　命中率的計算公式為:

　　Hit Ratio = 1 - ((physical reads - physical reads direct - physical reads direct (lob)) / (db block gets + consistent gets - physical reads direct - physical reads direct (lob))

　　分別代入上一查詢中的結(jié)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)數(shù)據(jù)緩沖區(qū)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';

　　這里命中率的計算應(yīng)該是

　　令 x = physical reads direct + physical reads direct (lob)

　　命中率 =100 - ( physical reads - x) / (consistent gets + db block gets - x)*100

　　通常如果發(fā)現(xiàn)命中率低于90%,則應(yīng)該調(diào)整應(yīng)用可可以考慮是否增大數(shù)據(jù)緩沖區(qū)

　　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%,就要考慮調(diào)整應(yīng)用(通常是沒使用bind var )或者增加內(nèi)存

　　6)計算在內(nèi)存中排序的比率:

　　SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';—查詢內(nèi)存排序數(shù) SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';—查詢磁盤排序數(shù) --caculate sort in memory ratio SQL>SELECT round(&sort_in_memory/(&sort_in_memory+&sort_in_disk),4)*100||'%' FROM dual;