转载  V$ACTIVE_SESSION_HISTORY视图相关了解---http://blog.csdn.net/haibusuanyun/article/details/17959973
V$ACTIVE_SESSION_HISTORY 显示数据库中的采样会话活动。ASH每秒从v$session中取快照，存在V$ACTIVE_SESSION_HISTORY中，并收集所有活动会话的等待信息。若ASH数据被刷新到磁盘，则需要从DBA_HIS_ACTIVE_SESS_HISTORY视图中查询相关信息。

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该视图是ASH的核心，用以记录活动SESSION的历史等待信息，每秒采样一次，这部分内容记录在内存中，期望值是记录一个小时的内容。

    --查找最近一分钟内，最消耗CPU的sql语句   
    SELECT sql_id,   
        count(*),   
        round(count(*) / sum(count(*)) over(), 2) pctload  
    FROM V$ACTIVE_SESSION_HISTORY  
    WHERE sample_time > sysdate - 1 / (24 * 60)  
        AND session_type <> 'BACKGROUND'  
        AND session_state = 'ON CPU'  
    GROUP BY sql_id  
    ORDER BY count(*) desc;  
      
    --查找最近一分钟内，最消耗I/O的sql语句  
    SELECT ash.sql_id,  
        count(*)  
    FROM V$ACTIVE_SESSION_HISTORY ASH,V$EVENT_NAME EVT  
    WHERE ash.sample_time > sysdate -1/(24*60)  
        AND ash.session_state = 'WAITING'  
        AND ash.event_id = evt.event_id  
        AND evt.wait_class = 'USER I/O'  
    GROUP BY ash.sql_id  
    ORDER BY count(*) desc;  
      
    --查找最近一分钟内，最消耗CPU的session  
    SELECT session_id,  
        count(*)  
        FROM V$ACTIVE_SESSION_HISTORY  
    WHERE session_state = 'ON CPU'  
        AND sample_time > sysdate -1/(24*60)  
    GROUP BY session_id  
    ORDER BY count(*) desc;  
      
    --查找最近一分钟内，最消耗资源的sql语句  
    SELECT ash.sql_id,  
        sum(decode(ash.session_state,'ON CPU',1,0)) "CPU",  
        sum(decode(ash.session_state,'WAITING',1,0)) -  
        sum(decode(ash.session_state,'WAITING',decode(en.wait_class,'USER I/O',1,0),0)) "WAIT",  
        sum(decode(ash.session_state,'WAITING',decode(en.wait_class,'USER I/O',1,0),0)) "IO",  
        sum(decode(ash.session_state,'ON CPU',1,1)) "TOTAL"  
        FROM V$ACTIVE_SESSION_HISTORY ASH,V$EVENT_NAME EN  
    WHERE SQL_ID is not null and en.event#=ash.event# and ash.sample_time > sysdate -1/(24*60)  
    GROUP BY ash.sql_id  
    ORDER BY sum(decode(ash.session_state,'ON CPU',1,1)) desc;  
      
    --查找最近一分钟内，最消耗资源的session  
    SELECT ash.session_id,  
        ash.session_serial#,  
        ash.user_id,  
        ash.program,  
        sum(decode(ash.session_state,'ON CPU',1,0)) "CPU",  
        sum(decode(ash.session_state,'WAITING',1,0)) -  
        sum(decode(ash.session_state,'WAITING',decode(en.wait_class,'USER I/O',1,0),0)) "WAITING",  
        sum(decode(ash.session_state,'WAITING',decode(en.wait_class,'USER I/O',1,0),0)) "IO",  
        sum(decode(ash.session_state,'ON CPU',1,1)) "TOTAL"  
    FROM V$ACTIVE_SESSION_HISTORY ASH,V$EVENT_NAME EN  
    WHERE en.event# = ash.event# and ash.sample_time > sysdate -1/(24*60)  
    GROUP BY ash.session_id,ash.user_id,ash.session_serial#,ash.program  
    ORDER BY sum(decode(ash.session_state,'ON CPU',1,1)); 


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