V$SYSTEM_EVENT、V$SESSION_EVENT、V$SESSION_WAIT&V$session、V$WAITSTAT

1、v$system_event

内容：统计数据，统计instance启动以来每种等待时间的等待次数、时间、平均时间等

目的：用来发现系统级评瓶颈

SQL> desc v$system_event

Name

------------------------

EVENT

TOTAL_WAITS --总等待次数

TOTAL_TIMEOUTS --等待超时次数

TIME_WAITED --总的等待时间，单位是百分之一秒 （最重要考虑）

AVERAGE_WAIT -- 平均等待时间，总的等待时间/总等待次数

TIME_WAITED_MICRO --总的等待时间的微秒形式 1/1000000

常见的等待现象：

latch free

enqueue

free buffer waits

buffer busy waits: 查看V$WAITSTAT

db file sequential read

db file scattered read

2、v$session_event（仅比v$system_event多一个sid字段）

内容：每个session所有等待事件的统计信息

目的：用来诊断缓慢原因

3、v$session或v$session_wait（当前session含 active和inactive）正在等待什么，无等待则显示上一次等待信息

SID: session id

? EVENT: Event the session is currently waiting for, or the last event the session had to wait for

? WAIT_TIME: Time (in hundredths of a second) that the session waited for the event; if the WAIT_TIME is 0, then the session is currently waiting for the event

? SEQ#: Gets incremented with every wait of the session

? P1, P2, P3: Wait event specific details for the wait

? P1TEXT, P2TEXT, P3TEXT: Description of P1,P2,P3 for the given event

WAIT_TIME Meaning Waiting

>0 Time waited in the last wait (in 10 ms clock ticks) No

0 Session is currently waiting for this event Yes

-1 Time waited in the last wait was less than 10 ms No

-2 Timing is not enabled No

4、V$WAITSTAT（统计）

目的：a summary all block buffer waits since instance startup。适于查找buffer busy waits