awr报告中Library Hit %显示负数

刘泽宇

"Library Hit %" Statistic Displays Negative Value in AWR Report (Doc ID 1604214.1)
现象为：
Instance Efficiency Percentages (Target 100%) section contains a negative Percentage value for the  "Library Hit %" statistic in an AWR report.
报告中的Efficiency Percentages部分Library Hit %数据为负值百分比

原因为：
The variable used to store this statistic is sized at 2^32. If a number is so large that it exceeds this then it 'wraps' and can display negative numbers. If figures in a prior snapshot are extremely high and then extremely low in the next, then it is possible to see negative values. This can be particularly apparent if the database was shutdown in the snapshots that are being compared. If the database was not re-started, then you can assume that any negative values are at least 2^32.
用于存储此统计信息的变量大小为2^32。如果某个数字太大以至于超过此值，则它会“换行”并显示负数。如果前一张快照中的数字非常高，然后下一张快照的数字非常低，则有可能看到负值。如果正在比较的快照中关闭了数据库，这一点尤其明显。如果数据库未重新启动，则可以假设任何负值至少为2^32。

目前暂无处理方法，如果不同时期的负载差异很大，请尽量不要比较这些快照。