Read by other session 等待说明

http://blog.csdn.net/tianlesoftware/article/details/7734551

在AWR中，我们可能会看到如下信息：

这里是Read byother session 等待占用了很多的时间。

关于Read by other session 的说明如下：

When informationis requested from the database, Oracle will first read the data from disk intothe database buffer cache. If two or more sessions request the sameinformation, the first session will read the data into the buffer cache whileother sessions wait. In previous versions this wait was classified under the"buffer busy waits" event. However, in Oracle 10.1 and higher thiswait time is now broken out into the "read by other session" waitevent. Excessive waits for this event are typically due to several processesrepeatedly reading the same blocks, e.g. many sessions scanning the same indexor performing full table scans on the same table. Tuning this issue is a matterof finding and eliminating this contention.

--当我们请求一个数据时，Oracle 第一次会从磁盘将数据读入buffer cache。如果有两个或者多个session 请求相同的信息，那么第一个session 会将这个信息读入buffer cache，其他的session 就会出现等待。 在10g之前，该等待事件还是在buffer busy waits 之下，在Oracle 10g之后，单独将该事件拿出并命令为read by other session。

一般来说出现这种等待事件是因为多个进程重复的读取相同的blocks，比如一些session 扫描相同的index或者在相同的block上执行full table scan。解决这个等待事件最好是找到并优化相关的SQL语句。

       Readby other session 等待的出现也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。