Optimizer does not identify index as covering on myisam table

In some cases optimizer fetches full rows even when the index used covers the entire query. The index-only plan is used when the index is forced by USE/FORCE INDEX() hint. If the order of columns in the index is changed, the query is executed index-only without the hint (but in the test case that leads to filesort).

Affects MySQL too (tested 5.6, probably others), reported as http://bugs.mysql.com/bug.php?id=81601