mysql-filesort-分析
https://zhuanlan.zhihu.com/p/311933050
MySQL :: MySQL 5.7 Reference Manual :: 8.2.1.14 ORDER BY Optimization
Use of filesort to Satisfy ORDER BY
If an index cannot be used to satisfy an
ORDER BY
clause, MySQL performs afilesort
operation that reads table rows and sorts them. Afilesort
constitutes an extra sorting phase in query execution.To obtain memory for
filesort
operations, the optimizer allocates a fixed amount of