Created_tmp_files ... What is the Cause?

Looking at a query that triggers a filesort (modified filesort method), I’m seeing the following:

±------------------------±------+| Created_tmp_disk_tables | 0 || Created_tmp_files | 2 || Created_tmp_tables | 0 |±------------------------±------+

Normally in memory filesorts use heap tables in-memory until they get too big. I’m not seeing that here.

This server is not involved in replication, so those tmp files are not the issue.

So what exactly causes Created_tmp_files and are these tmp files in memory or disk?

  1. in memory
  2. unindexed group by/order by most likely