如何计算几个命中率
(1)LC的命中率:
.计算公式:Library Cache Hit Ratio = sum(pinhits) / sum(pins)
SELECTSUM(pinhits)/sum(pins)
FROM V$LIBRARYCACHE
通常在98%以上，否则，需要要考虑加大共享池，绑定变量，修改cursor_sharing等参数。
.计算共享池内存使用率:
SELECT(1- ROUND(BYTES /(&TSP_IN_M *1024*1024),2))*100|| '%'
FROM V$SGASTAT
WHERENAME= '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:
SELECT*FROM V$SGASTAT
(2)PGA的命中率:
计算公式:BP x 100 / (BP + EBP)
BP: bytes processed
EBP: extrabytesread/written
SELECT*FROM V$PGASTAT WHERENAME='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:
SELECT round(PGA_TARGET_FOR_ESTIMATE/1024/1024) target_mb,
ESTD_PGA_CACHE_HIT_PERCENTAGE cache_hit_perc,
ESTD_OVERALLOC_COUNT
FROM V$PGA_TARGET_ADVICE;
The output of this query might look like the following:
TARGET_MB CACHE_HIT_PERC ESTD_OVERALLOC_COUNT
---------- -------------- --------------------
63 23 367
125 24 30
250 30 3
375 39 0
500 58 0
600 59 0
700 59 0
800 60 0
900 60 0
1000 61 0
1500 67 0
2000 76 0
3000 83 0
4000 85 0
在此例中:PGA至少要分配375M
我个人认为PGA命中率不应该低于50%
以下的SQL统计sql语句执行在三种模式的次数: optimal memory size, one-pass memory size, multi-pass memory size:
SELECT name profile, cnt, decode(total, 0, 0, round(cnt*100/total,4)) percentage
FROM (SELECT name, value cnt, (sum(value) over ()) total FROM V$SYSSTAT WHERE name like 'workarea exec%');
计算在内存中排序的比率:
SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)'—查询内存排序数
--
SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)'—查询磁盘排序数
--caculate sort in memory ratio
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
(3)db buffer cache命中率:
计算公式:hit ratio = 1 - [physical reads/(block gets + consistent gets)]
SELECT NAME, PHYSICAL_READS, DB_BLOCK_GETS, CONSISTENT_GETS,
1 - (PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS)) "Hit Ratio"
FROM V$BUFFER_POOL_STATISTICS
WHERE NAME='DEFAULT';
通常应在90%以上，否则，需要调整,加大DB_CACHE_SIZE
另外一种计算命中率的方法(摘自ORACLE官方文档<<数据库性能优化>>):
SELECT NAME, VALUE
FROM V$SYSSTAT
WHERE NAME IN('session logical reads',
'physical reads',
'physical reads direct',
'physical reads direct (lob)',
'db block gets', 'consistent gets');
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
-----------------------------------------------------------------------------
几个常用的检查语句
1. 查找排序最多的SQL:
SELECT HASH_VALUE, SQL_TEXT, SORTS, EXECUTIONS
FROM V$SQLAREA
ORDER BY SORTS DESC;
2. 查找磁盘读写最多的SQL:
SELECT * FROM
(SELECT sql_text,disk_reads "total disk" ,
executions "total exec",disk_reads/executions "disk/exec" FROM v$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):
select
substr(to_char(s.pct, '99.00'), 2) || '%' load,
s.executions executes,
p.sql_text
from
(
address,
disk_reads,
executions,
pct,
rank() over (order by disk_reads desc) ranking
100 * ratio_to_report(disk_reads) over () pct
sys.v_$sql
where
command_type != 47
)
disk_reads > 50 * executions
) s,
sys.v_$sqltext p
s.ranking <= 5 and
p.address = s.address
order by
1, s.address, p.piec
4.用下列SQL工具找出低效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;
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