最近一个生产库里出现大量db file scattered read和read by other session的等待，看了一下相关sql，都是对一个大表做全表扫描，10g官方文档里关于read by other session的解释如下：
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
可见，在10g以前的版本中该事件表现为buffer busy wait，而10g中关于buffer busy wait等待的说明似乎还没有更新：
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
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