When checking for applicability of join cache
we must disable its usage only if there is no
temp table in use.
When a temp table is used we can use join
cache (and it will not make the result-set 
unordered) to fill the temp table. The filesort() 
operation is then applied to the data in the temp 
table and hence is not affected by join cache
usage.
Fixed by narrowing the condition for disabling 
join cache to exclude the case where temp table
is used.