1、 extra用于存放无法归入其他类别的关键附加信息。 2、 当extra字段显示为Using filesort时，表示MySQL未利用表内索引顺序读取数据，而是通过外部排序方式对结果进行排序，这种操作被称为文件排序。由于该过程效率较低，会显著影响查询性能，属于需要重点关注 ...

One of the ways we measure applications is through performance. One of the metrics for application performance is user experience, which generally translates to “did the user need to wait longer than ...