MySQL EXPLAIN中的filesort是什么？

最开始我一为filesort是外排。结果不是，EXPLAIN中这个提示非常具有误导性。只要排序的时候不能用上索引，就会显示成filesort。 MySQL所谓的”filesort”进行的动作：

Read the rows that match the WHERE clause
For each row, store in the sort buffer a tuple consisting of the sort key value and the columns referenced by the query
When the sort buffer becomes full, sort the tuples by sort key value in memory and write it to a temporary file
After merge-sorting the temporary file, retrieve the rows in sorted order, but read the columns required by the query directly from the sorted tuples rather than by accessing the table a second time.

注意，当内存放不下所有的数据时，数据会被分块，对于每个块应用快排，对于多个块应用合并排序。