oracle性能调优：Oracle调优命中率这些你都知道吗？熟练的掌握一些知识，对自己是有很大帮助的，下面小编分享给大家。
1)共享区字典缓存区命中率
计算公式：SUM(gets - getmisses - usage -fixed) / SUM(gets)
命中率应大于0.85
SQL>select sum(gets-getmisses-usage-fixed)/sum(gets)
from v$rowcache;
2)数据高速缓存区命中率
计算公式：1-(physical reads / (db block gets + consistent gets))
命中率应大于0.90最好
SQL>select name,value
from v$sysstat
where name in ('physical reads','db block gets','consistent gets');
3)共享区库缓存区命中率
计算公式：SUM(pins - reloads) / SUM(pins)
命中率应大于0.99
SQL>select sum(pins-reloads)/sum(pins)
from v$librarycache;
4)检测回滚段的争用
SUM(waits)值应小于SUM(gets)值的1%
SQL>select sum(gets)，sum(waits)，sum(waits)/sum(gets)
from v$rollstat;
5)检测回滚段收缩次数
SQL>select name,shrinks
from v$rollstat, v$rollname
where v$rollstat.usn = v$rollname.usn;
几个常用的检查语句
1. 查找排序最多的SQL:
SQL>SELECT HASH_VALUE, SQL_TEXT, SORTS, EXECUTIONS
FROM V$SQLAREA
ORDER BY SORTS DESC;
2.查找磁盘读写最多的SQL:
SQL>SELECT * FROM
(SELECT sql_text,disk_reads "total disk" , executions "total exec",disk_reads/executions "disk/exec" FROM v$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(实际上也是按磁盘读写来排序的)：
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$sqlarea
Where executions>0 and buffer_gets >0 and (buffer_gets-disk_reads)/buffer_gets<0.8
Order by 4 desc;
5、根据sid查看对应连接正在运行的sql
SQL>select /*+ push_subq */command_type,sql_text,sharable_mem,persistent_mem,
runtime_mem,sorts,version_count,loaded_versions,open_versions,users_opening,executions,
users_executing,loads,first_load_time,invalidations,parse_calls,disk_reads,buffer_gets,
rows_processed,sysdate start_time,sysdate finish_time,'>'||address sql_address,'N'status
From v$sqlarea
Where address=(select sql_address from v$session where sid=&sid);
***************Oracle 缓冲区命中率低的分析及解决办法******************
首先确定下面的查询结果：
1,缓冲区命中率的查询(是否低于90%)：
select round((1 - sum(decode(name,'physical reads',value,0)) /
(sum(decode(name,'db block gets',value,0)) + sum(decode(name,'consistent gets',value,0))) )，4) *100 || '%' chitrati
from v$sysstat;
2,使用率的查询(有无free状态的数据快。)：
select count(*)， status from v$bh group by status ;
3,相关等待事件的查询(是否有相关等待事件)
select event,total_waits from v$system_event where event in ('free buffer waits');
4,当前大小(是否已经很大)
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$system_event)
order by total_waits desc)
where rownum<6
and event not like 'rdbms%'
and event not like 'pmon%'
and event not like 'SQL*Net%'
and event not like 'smon%';
6,db_cache_advice建议值(9i后的新特性，可以根据他更好的调整cache_size)
select block_size,size_for_estimate,size_factor,estd_physical_reads from v$db_cache_advice;
以上内容是关于java新手必看：经验总结oracle性能调优：Oracle调优命中率的介绍，要想了解更多相关信息、教育培训内容，请随时关注唯学网，小编会第一时间为大家更新、跟进最新信息。
