sys库说明：http://dev.mysql.com/doc/refman/5.7/en/sys-schema-usage.html
sys库使用说明：http://dev.mysql.com/doc/refman/5.7/en/sys-schema-object-index.html
参考：http://www.cnblogs.com/jiangwenju/p/5157333.html
sys系统数据库结合了information_schema和performance_schema的数据，方便管理者和使用者查看数据库实例运行和使用情况。
打开sys库后可以看到一个表和99个视图，视图有两种，带'x$'的视图检出的数据没有经过转换，不带'x$'的视图检出的数据经过转换便于管理员直接阅读使用。官网的说明如下：
The view without the x$ prefix is intended to provide output that is more user friendly and easier for humans to read. The view with the x$ prefix that displays the same values in raw form is intended more for use with other tools that perform their own processing on the data. For additional information about the differences between non-x$ and x$ views, see Section 24.4., “sys Schema Views”.