Oracle数据库调试和优化详解

此文主要是关于Oracle数据库调试和优化方面的相关内容的介绍，还有涉及到Oracle数据库中命里率的相关问题的解答，其中包括不同的算法之间性能的比对。以下就是相关内容的介绍。

关于Oracle中各个命中率的计算以及相关的调优

1)Library Cache的命中率:

计算公式:

1. Library Cache Hit Ratio = sum(pinhits) / sum(pins)

2. 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:

1. 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:

1. SQL>SELECT * FROM V$SGASTAT WHERE NAME =
'free memory' AND POOL = 'shared pool';

3)db buffer cache命中率:

计算公式:

1.Hit ratio = 1 - [physical reads/(block gets + consistent gets)]

2.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<>):命中率的计算公式为:数据库性能优化>

1. 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

1.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：

1. 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

1. x = physical reads direct + physical reads direct (lob)

命中率

1. =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：

1. 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:

1.SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';

—查询内存排序数

1.SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';

—查询磁盘排序数

1. --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

1.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:

1.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:

1.optimal memory size, one-pass memory size, multi-pass memory size:

2.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%');

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