(Initial patch by Varun Gupta. Amended and added comments).

When the query has both
1. Aggregate functions that require sorting data by group, and
2. Window functions

we need to use two temporary tables. The first temp.table will hold the
join output.  Then it is passed to filesort(). Reading it in sorted
order allows to compute the aggregate functions.

Then, we need to write their values into the second temp. table. Then,
Window Function computation step can pass that to filesort() and read
them in the order it needs.

Failure to create the second temp. table would cause an assertion
failure: window function could would not find where to get the values
of the aggregate functions.