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本文是關于Oracle數據庫調試與優化方面的文章，主要介紹Oracle數據庫中命中率相關的問題，包括不同的算法之間性能的比對。51CTO編輯向您推薦《Oracle數據庫調試與性能優化》

關于Oracle中各個命中率的計算以及相關的調優

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。

2)計算共享池內存使用率:

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)

共享池內存使用率，應該穩定在75%-90%間，太小浪費內存，太大則內存不足。

查詢空閑的共享池內存:

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';

通常應在90%以上，否則，需要調整,加大DB_CACHE_SIZE

另外一種計算命中率的方法(摘自ORACLE官方文檔<<數據庫性能優化>>):

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)數據緩沖區命中率：

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

通常如果發現命中率低于90%,則應該調整應用可可以考慮是否增大數據緩沖區

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%,就要考慮調整應用（通常是沒使用bind var ）或者增加內存

6)計算在內存中排序的比率:

SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';—查詢內存排序數

SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';—查詢磁盤排序數

--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,太小整要考慮調整,加大PGA

7)PGA的命中率:

計算公式:BP x 100 / (BP + EBP)

BP: bytes processed

EBP: extra bytes read/written

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

或者從OEM的圖形界面中查看

我們可以查看一個視圖以獲取Oracle的建議值:

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

我個人認為PGA命中率不應該低于50%

以下的SQL統計sql語句執行在三種模式的次數: optimal memory size, one-pass memory size, multi-pass memory size:

SQL>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)共享區字典緩存區命中率

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

命中率應大于0.85

SQL>select sum(gets-getmisses-usage-fixed)/sum(gets)

from v$rowcache;

9)數據高速緩存區命中率

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

命中率應大于0.90最好

SQL>select name,value

from v$sysstat

where name in ('physical reads','db block gets','consistent gets');

10)共享區庫緩存區命中率

計算公式：SUM(pins - reloads) / SUM(pins)

命中率應大于0.99

SQL>select sum(pins-reloads)/sum(pins)

from v$librarycache;

11)檢測回滾段的爭用

SUM(waits)值應小于SUM(gets)值的1%

SQL>select sum(gets),sum(waits),sum(waits)/sum(gets)

from v$rollstat;

12)檢測回滾段收縮次數

SQL>select name,shrinks

from v$rollstat, v$rollname

where v$rollstat.usn = v$rollname.usn;

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

幾個常用的檢查語句

1. 查找排序最多的SQL:

SQL>SELECT HASH_VALUE, SQL_TEXT, SORTS, EXECUTIONS

FROM V$SQLAREA

ORDER BY SORTS DESC;

2.查找磁盤讀寫最多的SQL:

SQL>SELECT * FROM

(SELECT sql_text,disk_reads "total disk" , executions "total exec",disk_reads/executions "disk/exec" FROM v$sql

WHERE executions>0 and is_obsolete='N' ORDER BY 4 desc)

WHERE ROWNUM<11 ;

3.查找工作量最大的SQL(實際上也是按磁盤讀寫來排序的):

SQL>select substr(to_char(s.pct, '99.00'), 2) || '%' load,s.executions executes,p.sql_text

from(select address,disk_reads,executions,pct,rank() over (order by disk_reads desc) ranking from

(select address,disk_reads,executions,100 * ratio_to_report(disk_reads) over () pct

from sys.v_$sql

where command_type != 47)

where disk_reads > 50 * executions) s,sys.v_$sqltext p

where s.ranking <= 5 and p.address = s.address

order by 1, s.address, p.piece;

4. 用下列SQL工具找出低效SQL:

SQL>select executions,disk_reads,buffer_gets,round((buffer_gets-disk_reads)/buffer_gets,2) Hit_radio,round(disk_reads/executions,2) reads_per_run,sql_text

From v$sqlarea

Where executions>0 and buffer_gets >0 and (buffer_gets-disk_reads)/buffer_gets<0.8

Order by 4 desc;

5、根據sid查看對應連接正在運行的sql

SQL>select /*+ push_subq */command_type,sql_text,sharable_mem,persistent_mem,

runtime_mem,sorts,version_count,loaded_versions,open_versions,users_opening,executions,

users_executing,loads,first_load_time,invalidations,parse_calls,disk_reads,buffer_gets,

rows_processed,sysdate start_time,sysdate finish_time,’>’||address sql_address,’N’status

From v$sqlarea

Where address=(select sql_address from v$session where sid=&sid);

***************Oracle 緩沖區命中率低的分析及解決辦法******************

首先確定下面的查詢結果:

1,緩沖區命中率的查詢(是否低于90%):

select round((1 - sum(decode(name,'physical reads',value,0)) /

(sum(decode(name,'db block gets',value,0)) + sum(decode(name,'consistent gets',value,0))) ),4) *100 || '%' chitrati

from v$sysstat;

2,使用率的查詢(有無free狀態的數據快.):

select count(*), status from v$bh group by status ;

3,相關等待事件的查詢(是否有相關等待事件)

select event,total_waits from v$system_event where event in ('free buffer waits');

4,當前大小(是否已經很大)

select value/1024/1024 cache_size from v$parameter where name='db_cache_size'

5,top等待事件分析(Db file scatered read的比率是否大)

select event ,total_waits,suml

from

(select event,total_waits,round(total_waits/sumt*100,2)||'%' suml

from

(select event,total_waits from v$system_event ),

(select sum(total_waits) sumt from v$system_event)

order by total_waits desc)

where rownum<6

and event not like 'rdbms%'

and event not like 'pmon%'

and event not like 'SQL*Net%'

and event not like 'smon%';

6,db_cache_advice建議值（9i后的新特性,可以根據他更好的調整cache_size）

select block_size,size_for_estimate,size_factor,estd_physical_reads from v$db_cache_advice;

說明分析:

緩沖區命中率(低于90的命中率就算比較低的).

沒有free不一定說明需要增加,還要結合當前cache_size的大小(我們是否還可以再增大,是否有需要增加硬件,增加開銷),

空閑緩沖區等待說明進程找不到空閑緩沖區，并通過寫出灰緩沖區，來加速數據庫寫入器生成空閑緩沖區，當DBWn將塊寫入磁盤后，灰數據緩沖區將被釋放，以便重新使用.產生這種原因主要是: