Bug #29717 INSERT INTO SELECT inserts values even if

SELECT statement itself returns empty.

As a result of this bug 'SELECT AGGREGATE_FUNCTION(fld) ... GROUP BY'
can return one row instead of an empty result set.

When GROUP BY only has fields of constant tables
(with a single row), the optimizer deletes the group_list.
After that we lose the information about whether we had an
GROUP BY statement. Though it's important
as SELECT min(x) from empty_table; and
   SELECT min(x) from empty_table GROUP BY y; have to return
different results - the first query should return one row,
second - an empty result set.
So here we add the 'group_optimized_away' flag to remember this case
when GROUP BY exists in the query and is removed
by the optimizer, and check this flag in end_send_group()

sql/sql_select.h

@@ -180,6 +180,14 @@ class JOIN :public Sql_alloc
   ROLLUP rollup;				// Used with rollup
 
   bool select_distinct;				// Set if SELECT DISTINCT
+  /*
+    If we have the GROUP BY statement in the query,
+    but the group_list was emptied by optimizer, this
+    flag is TRUE.
+    It happens when fields in the GROUP BY are from
+    constant table
+  */
+  bool group_optimized_away;
 
   /*
     simple_xxxxx is set if ORDER/GROUP BY doesn't include any references
@@ -276,6 +284,7 @@ class JOIN :public Sql_alloc
     ref_pointer_array_size= 0;
     zero_result_cause= 0;
     optimized= 0;
+    group_optimized_away= 0;
 
     fields_list= fields_arg;
     bzero((char*) &keyuse,sizeof(keyuse));