10g wait event: read by other session
在一台10g的数据库上无意中看v$session_wait时看到这个event，这个wait event在9i里面是没有的
10g中将很多wait event都细化了，同样的，这个event来自于buffer busy waits
查了下官方文档：
read by other session
This event occurs when a session requests a buffer that is currently being read into the buffer cache by another session. Prior to release 10.1, waits for this event were grouped with the other reasons for waiting for buffers under the ‘buffer busy wait’ event
不过看看buffer busy waits的解释还没有更新，没有将第一个原因分离出来
buffer busy waits
Wait until a buffer becomes available. This event happens because a buffer is either being read into the buffer cache by another session (and the session is waiting for that read to complete) or the buffer is the buffer cache, but in a incompatible mode (that is, some other session is changing the buffer).
到10g的数据库上看，大部分等待来自于第一个原因
- SQL> select * from v$version;
- BANNER
- ----------------------------------------------------------------
- Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bi
- PL/SQL Release 10.2.0.3.0 - Production
- CORE 10.2.0.3.0 Production
- TNS for Solaris: Version 10.2.0.3.0 - Production
- NLSRTL Version 10.2.0.3.0 - Production
- SQL> select event,total_waits,total_timeouts from v$system_event where event in ('buffer busy waits','read by other session');
- EVENT TOTAL_WAITS TOTAL_TIMEOUTS
- ------------------------------------------------------------ ----------- --------------
- buffer busy waits 639188 0
- read by other session 5334877 20
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