Oracle为我们提供了获取这些等待事件的可用视图。根据这些视图可以得知哪些事件导致该SQL语句效率低下而采取相应的修改或调整。本
通常情况下,用户提交一条SQL语句,总会存在这样或那样的等待事件。也就是说由于所需资源被占用导致进程不得不处于等待状态。Oracle为我们提供了获取这些等待事件的可用视图。根据这些视图可以得知哪些事件导致该SQL语句效率低下而采取相应的修改或调整。本文基于Oracle 10g描述了如何通过视图v$session_wait,v$session_event,以及v$system_event去获取等待事件的相关信息。
1、等待事件相关参数
timed_statistics
指定了在搜集数据库统计信息时,与时间相关的统计信息是否会被收集。该参数影响等待事件中时间相关的值,且受制于statistics_level。
statistics_level
指定了收集数据库以及操作系统统计信息的级别。
该参数值的范围STATISTICS_LEVEL = { ALL | TYPICAL | BASIC },可修改级别ALTER SESSION, ALTER SYSTEM
缺省情况下为TYPICAL
2、等待事件的相关视图
可以从dict数据库字典中获得当前数据库的与等待事件相关的视图,如使用如下SQL来获得:
SELECT *
FROM dict
WHERE table_name LIKE '%V$EVENT%' OR table_name LIKE '%V$%WAIT%';
几个重要的等待事件视图
下面这三个视图提供了不同粒度级的等待事件统计和计时信息。
v$SESSION_WAIT(最低粒度的等待事件)
v$SESSION_EVENT(该视图提供上一视图等待事件的汇总)
v$SYSTEM_EVENT(该视图提供自实例启动以来所有等待事件的汇总)
几个视图的粒度关系
V$SESSION_WAIT ⊂ v$SESSION_EVENT ⊂ v$SYSTEM_EVENT
下面将逐一描述这些视图
3、视图v$event_name
该视图列出了当前Oracle版本中所有等待事件的描述信息。如等待事件的ID,,名字,参数,类别等等
SELECT COUNT (*) FROM v$event_name;
SELECT wait_class, COUNT (*)
FROM v$event_name
GROUP BY wait_class
ORDER BY 2;
4、视图v$session_wait
该视图显示的基于会话级的等待信息,为每个会话当前正在等待的事件或资源提供详细信息。
也就是说在视图v$session_wait中,每一个连接到的实例的session都对应一行活动的或不活动的信息。
视图的p1,p2,p3标识了对应的session正在等待的具体资源,p1text,p2text,p3text则是对p1,p2,p3的文字描述。
从Oracle 10g R1启,该视图的所有信息可以直接从v$session获得。
1)、state字段有四种含义﹕
a、Waiting:当前SESSION正等待这个事件。SECONDS_IN_WAIT表示已经等待的时间。
b、Waited unknown time:参数timed_statistics值为false时,无法确定等待时间。
c、Wait short time:表示发生了等待,且低于1厘秒,此情形下wait_time列显示为-1。
d、Waited known time:如果session等待然后得到了所需资源,那么将从waiting进入本状态,并且时间发送给wait_time列
2)、wait_time值也有四种含义:
a、值>0:最后一次等待时间(单位:厘秒),当前未在等待状态。
b、值=0:session正在等待当前的事件。
c、值=-1:最后一次等待时间小于1个统计单位,当前未在等待状态。
d、值=-2:时间统计状态未置为可用,当前未在等待状态。
3)、wait_time和seconds_in_wait字段值与state相关:
a、如果state值为Waiting,则wait_time值无效。seconds_in_wait值为实际的等待时间(单位:秒)。
b、如果state值为Wait unknow time或者Wait short time,那么wait_time值和Seconds_in_wait值都无效。
c、如果state值为Waiting known time,则wait_time值就是实际等待时间(单位:秒),seconds_in_wait值无效。
4)、指定session当前的等待事件
SELECT *
FROM v$session_wait
WHERE sid = &input_sid;

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