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$SGASTAT WHERE NAME = ‘free memory’ AND POOL = ‘shared pool’;
3)db buffer cache命中率:
计算公式:Hit ratio = 1 – [physical reads/(block gets + consistent gets)]
通常应在90%以上，否则，需要调整,加大DB_CACHE_SIZE
另外一种计算命中率的方法(摘自Oracle官方文档):
命中率的计算公式为:
Hit Ratio = 1 – ((physical reads – physical reads direct – physical reads direct (lob)) / (db block gets + consistent gets – physical reads direct – physical reads direct (lob))
分别代入上一查询中的结果值,就得出了Buffer cache的命中率
SQL>SELECT NAME, VALUE FROM V$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:
–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
———- ————– ——————–
在此例中:PGA至少要分配375M
我个人认为PGA命中率不应该低于50%
以下的SQL统计sql语句执行在三种模式的次数: 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;
9)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$sysstat where name in (‘physical reads’,’db block gets’,’consistent gets’);
10)共享区库缓存区命中率
计算公式：SUM(pins – reloads) / SUM(pins)
命中率应大于0.99
SQL>select sum(pins-reloads)/sum(pins) from v$librarycache;
11)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$rollstat;
12)检测回滚段收缩次数
SQL>select name,shrinks from v$rollstat, v$rollname where v$rollstat.usn = v$rollname.usn;
1. 查找排序最多的SQL:
SQL>SELECT HASH_VALUE, SQL_TEXT, SORTS, EXECUTIONS FROM V$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);
首先确定下面的查询结果:
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$sysstat;
select count(*), status from v$bh group by status;
select event,total_waits from v$system_event where event in (‘free buffer waits’);
select value/1024/1024 cache_size from v$parameter where name=’db_cache_size’;
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%';
select block_size,size_for_estimate,size_factor,estd_physical_reads from v$db_cache_advice;
说明分析:
缓冲区命中率(低于90的命中率就算比较低的).
没有free不一定说明需要增加,还要结合当前cache_size的大小(我们是否还可以再增大,是否有需要增加硬件,增加开销),
空闲缓冲区等待说明进程找不到空闲缓冲区，并通过写出灰缓冲区，来加速数据库写入器生成空闲缓冲区，当DBWn将块写入磁盘后，灰数据缓冲区将被释放，以便重新使用.产生这种原因主要是:
数据分散读等待,通常表现存在着与全表扫描相关的等待，逻辑读时，在内存中进行的全表扫描一般是零散地，而并非连续的被分散到缓冲区的各个部分，可能有索引丢失，或被仰制索引的存在。该等待时间在数据库会话等待多块io读取结束的时候产生，并把指定的块数离散的分布在数据缓冲区。这意味这全表扫描过多，或者io不足或争用，
存在这个事件,多数都是问题的,这说明大量的全部扫描而未采用索引。
db_cache_advice对我们调整db_cache_size大小有一定的帮助,但这只是一个参考，不一定很精确。
通过上面6种情况的综合分析,判断是否需要增加大cache_size. 或者把常用的(小)表放到keep区。
但多数的时候做这些不会解决质的问题,而真正的问题主要是对sql语句的优化(如:是否存在大量的全表扫描等)索引是在不需要改变程序的情况下,对数据库性能，sql语句提高的最实用的方法.
我在生产中遇到过类似的问题,200M的cache_size,命中率很低21%,但通过对sql语句的优化(添加索引,避免全表扫描),命中率增加到96%,程序运行时间由原来的2小时减少到不到10分钟.
这就提到了怎么定位高消耗的sql问题.全表扫描的问题,在这里不做细致的解说,这里只说明方法,我会在相关的章节专门介绍怎么使用这些工具
1.sql_trace跟踪session.用tkprof 分别输出磁盘读,逻辑读,运行时间长的sql进行优化.这些高消耗的sql一般都伴随着全表扫描.
2.statspack分析.在系统繁忙时期进行时间点的统计分析,产看TOP事件是否有Db file scatered read.并查看TOP sql语句是否存在问题等.
1、查看Oracle数据库缓冲区命中率
select a.value + b.value “logical_reads”, c.value “phys_reads”, round(100 * ((a.value+b.value)-c.value) / (a.value+b.value)) “BUFFER HIT RATIO” from v$sysstat a, v$sysstat b, v$sysstat c where a.statistic# = 40 and b.statistic# = 41 and c.statistic# = 42;
2、Tags: oracle
数据库缓冲区命中率：
sql>select value from v$sysstat where name =’physical reads’; value 3714179 sql>select value from v$sysstat where name =’physical reads direct’; value 0 sql>select value from v$sysstat where name =’physical reads direct(lob)’; value 0 sql>select value from v$sysstat where name =’consistent gets’; value 856309623 sql>select value from v$sysstat where name =’db block gets’; value 19847790
这里命中率的计算应该是
令x=physical reads direct + physical reads direct(lob)
命中率=100-(physical reads -x)/(consistent gets +db block gets -x)*100
通常如果发现命中率低于90%，则应该调整应用可以考虑是否增大数据加
共享池的命中率
sql> select sum(pinhits)/sum(pins)*100 “hit radio” from v$librarycache;
如果共享池的命中率低于95%就要考虑调整应用(通常是没应用bind var)或者增加内存。
关于排序部分
sql> select name,value from v$sysstat where name like ‘%sort%’;
如果我们发现sorts(disk)/(sorts(memory)+sorts(disk))的比例过高，则通常意味着sort_area_size部分内存教较小，可考虑调整相应的参数。
关于log_buffer
sql>select name,value from v$sysstat where name in (‘redo entries’,’redo buffer allocation retries’);
假如redo buffer allocation retries/redo entries的比例超过1%我们就可以考虑增加log_buffer.