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.