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$SGASTAT
WHERE NAME = 'free memory' AND POOL = 'shared pool';
3)db buffer cache命中率:
计算公式:
Hit ratio = 1 - [physical reads/(block gets + consistent gets)]
SQL>SELECT NAME, PHYSICAL_READS, DB_BLOCK_GETS, CONSISTENT_GETS,
1 - (PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS)) "Hit Ratio"
FROM V$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<<数据库性能优化>>):
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$librarycache;
假如共享池的命中率低于95%,就要考虑调整应用（通常是没使用bind var ）或者增加内存
6)计算在内存中排序的比率:
SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';—查询内存排序数
SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';—查询磁盘排序数
--caculate sort in memory ratio
SQL>SELECT round(&sort_in_memory/(&sort_in_memory+&sort_in_disk),4)*100||'%' FROM dual;
此比率越大越好,太小整要考虑调整,加大PGA
7)PGA的命中率:
计算公式:
BP x 100 / (BP + EBP)
BP: bytes processed
EBP: extra bytes read/written
SQL>SELECT * FROM V$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$PGA_TARGET_ADVICE;
The output of this query might look like the following:
TARGET_MB CACHE_HIT_PERC ESTD_OVERALLOC_COUNT
---------- -------------- --------------------
63 23 367
125 24 30
250 30 3
375 39 0
500 58 0
600 59 0
700 59 0
800 60 0
900 60 0
在此例中:PGA至少要分配375M
我个人认为PGA命中率不应该低于50%
以下的SQL统计sql语句执行在三种模式的次数: optimal memory size, one-pass memory size, multi-pass memory size:
SQL>SELECT name profile, cnt, decode(total, 0, 0, round(cnt*100/total,4)) percentage
FROM (SELECT name, value cnt, (sum(value) over ()) total FROM V$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;
-----------------------------------------------------------------------------
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);
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