Bug #27807.

Non-correlated scalar subqueries may get executed
in EXPLAIN at the optimization phase if they are
part of a right hand sargable expression.
If the scalar subquery uses a temp table to 
materialize its results it will replace the 
subquery structure from the parser with a simple
select from the materialization table.
As a result the EXPLAIN will crash as the 
temporary materialization table is not to be shown
in EXPLAIN at all.
Fixed by preserving the original query structure
right after calling optimize() for scalar subqueries
with temp tables executed during EXPLAIN.

mysql-test/r/subselect.result

@@ -4012,3 +4012,11 @@ WHERE (SELECT COUNT(t0.b) FROM t1 t WHERE t.b>20) GROUP BY a;
 ERROR HY000: Invalid use of group function
 SET @@sql_mode=default;
 DROP TABLE t1;
+CREATE TABLE t1 (a int, b int, KEY (a));
+INSERT INTO t1 VALUES (1,1),(2,1);
+EXPLAIN SELECT 1 FROM t1 WHERE a = (SELECT COUNT(*) FROM t1 GROUP BY b);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ref	a	a	5	const	1	Using where; Using index
+2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using temporary; Using filesort
+DROP TABLE t1;
+End of 5.0 tests.

mysql-test/t/subselect.test

@@ -2845,3 +2845,13 @@ SELECT a FROM t1 t0
 SET @@sql_mode=default;
 
 DROP TABLE t1;
+
+#
+# Bug #27807: Server crash when executing subquery with EXPLAIN
+#  
+CREATE TABLE t1 (a int, b int, KEY (a)); 
+INSERT INTO t1 VALUES (1,1),(2,1);
+EXPLAIN SELECT 1 FROM t1 WHERE a = (SELECT COUNT(*) FROM t1 GROUP BY b);
+DROP TABLE t1;
+
+--echo End of 5.0 tests.

sql/item_subselect.cc

@@ -1774,6 +1774,21 @@ int subselect_single_select_engine::exec()
       thd->lex->current_select= save_select;
       DBUG_RETURN(join->error ? join->error : 1);
     }
+    if (!select_lex->uncacheable && thd->lex->describe && 
+        !(join->select_options & SELECT_DESCRIBE) && 
+        join->need_tmp && item->const_item())
+    {
+      /*
+        Force join->join_tmp creation, because this subquery will be replaced
+        by a simple select from the materialization temp table by optimize()
+        called by EXPLAIN and we need to preserve the initial query structure
+        so we can display it.
+       */
+      select_lex->uncacheable|= UNCACHEABLE_EXPLAIN;
+      select_lex->master_unit()->uncacheable|= UNCACHEABLE_EXPLAIN;
+      if (join->init_save_join_tab())
+        DBUG_RETURN(1);
+    }
     if (item->engine_changed)
     {
       DBUG_RETURN(1);

sql/sql_select.cc

@@ -1426,14 +1426,13 @@ JOIN::optimize()
       }
     }
 
-    if (select_lex->uncacheable && !is_top_level_join())
-    {
-      /* If this join belongs to an uncacheable subquery */
-      if (!(tmp_join= (JOIN*)thd->alloc(sizeof(JOIN))))
+    /* 
+      If this join belongs to an uncacheable subquery save 
+      the original join 
+    */
+    if (select_lex->uncacheable && !is_top_level_join() &&
+        init_save_join_tab())
       DBUG_RETURN(-1);
-      error= 0;				// Ensure that tmp_join.error= 0
-      restore_tmp();
-    }
   }
 
   error= 0;
@@ -1495,6 +1494,27 @@ JOIN::reinit()
   DBUG_RETURN(0);
 }
 
+/**
+   @brief Save the original join layout
+      
+   @details Saves the original join layout so it can be reused in 
+   re-execution and for EXPLAIN.
+             
+   @return Operation status
+   @retval 0      success.
+   @retval 1      error occurred.
+*/
+
+bool
+JOIN::init_save_join_tab()
+{
+  if (!(tmp_join= (JOIN*)thd->alloc(sizeof(JOIN))))
+    return 1;
+  error= 0;				       // Ensure that tmp_join.error= 0
+  restore_tmp();
+  return 0;
+}
+
 
 bool
 JOIN::save_join_tab()

sql/sql_select.h

@@ -434,6 +434,7 @@ public:
   void cleanup(bool full);
   void clear();
   bool save_join_tab();
+  bool init_save_join_tab();
   bool send_row_on_empty_set()
   {
     return (do_send_rows && tmp_table_param.sum_func_count != 0 &&