Bug #43029: FORCE INDEX FOR ORDER BY is ignored when join

buffering is used

FORCE INDEX FOR ORDER BY now prevents the optimizer from 
using join buffering. As a result the optimizer can use
indexed access on the first table and doesn't need to 
sort the complete resultset at the end of the statement.

sql/sql_select.h

@@ -357,6 +357,8 @@ public:
     simple_xxxxx is set if ORDER/GROUP BY doesn't include any references
     to other tables than the first non-constant table in the JOIN.
     It's also set if ORDER/GROUP BY is empty.
+    Used for deciding for or against using a temporary table to compute 
+    GROUP/ORDER BY.
   */
   bool simple_order, simple_group;
   /**