Correction for the fix of the bug mdev-6874.

sql/item_subselect.cc

@@ -3397,7 +3397,8 @@ bool subselect_union_engine::no_rows()
   /* Check if we got any rows when reading UNION result from temp. table: */
   return MY_TEST(!(unit->fake_select_lex ?
                    unit->fake_select_lex->join->send_records :
-                   ((select_union_direct *) result)->send_records));
+                   ((select_union_direct *)(unit->get_union_result()))
+                                                            ->send_records));
 }
 
 

sql/sql_class.h

@@ -4463,13 +4463,7 @@ class select_union_direct :public select_union
   }
   void cleanup()
   {
-    /*
-      Only called for top-level select_results, usually select_send,
-      and for the results of subquery engines
-      (select_<something>_subselect).
-    */
     send_records= 0;
-    DBUG_ASSERT(false); /* purecov: inspected */
   }
   void set_thd(THD *thd_arg)
   {

sql/sql_lex.h

@@ -676,6 +676,7 @@ class st_select_lex_unit: public st_select_lex_node {
 
   List<Item> *get_unit_column_types();
 
+  select_union *get_union_result() { return union_result; }
   int save_union_explain(Explain_query *output);
   int save_union_explain_part2(Explain_query *output);
 };

sql/sql_union.cc

@@ -784,6 +784,8 @@ bool st_select_lex_unit::exec()
 
   if (uncacheable || !item || !item->assigned() || describe)
   {
+    if (!fake_select_lex)
+      union_result->cleanup();
     for (SELECT_LEX *sl= select_cursor; sl; sl= sl->next_select())
     {
       ha_rows records_at_start= 0;