Re-filing this here from https://bugzilla.suse.com/show_bug.cgi?id=949520 :
Consider a query:
The join optimizer picks the join order of sa, su.
Table sa has an index which allows to satisfy ORDER BY LIMIT clause:
Good so far.
Now, let's try to change ORDER BY sa.id into ORDER BY su.id. The query
has sa.id = su.id, both columns have identical data types, so there should
be no difference.
However, there is:
ORDER BY optimizer no longer recognizes that index on sa.id produces the desired ordering.