蔡骏天机 全本下载:read by other session

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read by other session是在Oracle Database 10g中，引入的一个新事件，此前版本，这个事件在Buffer Busy Wait中记录。

根据文档，这个事件的解释如下：
This event occurs when a session requests a buffer that is currently being read into the buffer cache 
by another session. Prior to release 10.1, waits for this event were grouped with the other reasons for 
waiting for buffers under the 'buffer busy wait' event

这个等待也就是说明数据库存在读的竞争，所以该等待事件通常和db file sequential read或db file scattered read 同时出现。

以下是一个客户生产数据库中的Top 5 Timed Events：
Top 5 Timed Events                                        Avg %Total
~~~~~~~~~~~~~~~~~~                                        wait  Call
Event                                Waits    Time (s)  (ms)  Time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
CPU time                                          4,897          52.9
db file sequential read          1,557,308      1,887      1  20.4  User I/O
read by other session            3,818,768      1,769      0  19.1  User I/O
db file scattered read              973,678        807      1    8.7  User I/O
log file sync    
可通过如下SQL语句查看竞争的数据块：
SELECT p1 “file#”,p2 “block#”,p3 “class#” FROM v$session_wait WHERE event = 'read by other session';
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