Bug#56814 Explain + subselect + fulltext crashes server

create_sort_index() function overwrites original JOIN_TAB::type field.
At re-execution of subquery overwritten JOIN_TAB::type(JT_ALL) is
used instead of JT_FT. It misleads test_if_skip_sort_order() and
the function tries to find suitable key for the order that should
not be allowed for FULLTEXT(JT_FT) table.
The fix is to restore JOIN_TAB strucures for subselect on re-execution
for EXPLAIN.
Additional fix:
Update TABLE::maybe_null field which
affects list_contains_unique_index() behaviour as it
could have the value(maybe_null==TRUE) based on the
assumption that this join is outer
(see setup_table_map() func).

mysql-test/r/explain.result

@@ -251,4 +251,50 @@ EXPLAIN SELECT c1 FROM t1 WHERE c2 = 1 AND c4 = 1 AND c5 = 1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ref	c2,c2_2	c2	10	const,const	3	Using where
 DROP TABLE t1;
+#
+# Bug#56814 Explain + subselect + fulltext crashes server
+#
+CREATE TABLE t1(f1 VARCHAR(6) NOT NULL,
+FULLTEXT KEY(f1),UNIQUE(f1));
+INSERT INTO t1 VALUES ('test');
+EXPLAIN SELECT 1 FROM t1
+WHERE 1 > ALL((SELECT 1 FROM t1 JOIN t1 a ON (MATCH(t1.f1) AGAINST (""))
+WHERE t1.f1 GROUP BY t1.f1));
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
+2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	Using filesort
+2	SUBQUERY	t1	fulltext	f1	f1	0		1	Using where
+PREPARE stmt FROM
+'EXPLAIN SELECT 1 FROM t1
+ WHERE 1 > ALL((SELECT 1 FROM t1 RIGHT OUTER JOIN t1 a
+ ON (MATCH(t1.f1) AGAINST (""))
+ WHERE t1.f1 GROUP BY t1.f1))';
+EXECUTE stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
+2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	Using filesort
+2	SUBQUERY	t1	fulltext	f1	f1	0		1	Using where
+EXECUTE stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
+2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	Using filesort
+2	SUBQUERY	t1	fulltext	f1	f1	0		1	Using where
+DEALLOCATE PREPARE stmt;
+PREPARE stmt FROM
+'EXPLAIN SELECT 1 FROM t1
+ WHERE 1 > ALL((SELECT 1 FROM t1 JOIN t1 a
+ ON (MATCH(t1.f1) AGAINST (""))
+ WHERE t1.f1 GROUP BY t1.f1))';
+EXECUTE stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
+2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	Using filesort
+2	SUBQUERY	t1	fulltext	f1	f1	0		1	Using where
+EXECUTE stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
+2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	Using filesort
+2	SUBQUERY	t1	fulltext	f1	f1	0		1	Using where
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;
 End of 5.1 tests.

mysql-test/t/explain.test

@@ -228,4 +228,40 @@ EXPLAIN SELECT c1 FROM t1 WHERE c2 = 1 AND c4 = 1 AND c5 = 1;
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug#56814 Explain + subselect + fulltext crashes server
+--echo #
+
+CREATE TABLE t1(f1 VARCHAR(6) NOT NULL,
+FULLTEXT KEY(f1),UNIQUE(f1));
+INSERT INTO t1 VALUES ('test');
+
+EXPLAIN SELECT 1 FROM t1
+WHERE 1 > ALL((SELECT 1 FROM t1 JOIN t1 a ON (MATCH(t1.f1) AGAINST (""))
+WHERE t1.f1 GROUP BY t1.f1));
+
+PREPARE stmt FROM
+'EXPLAIN SELECT 1 FROM t1
+ WHERE 1 > ALL((SELECT 1 FROM t1 RIGHT OUTER JOIN t1 a
+ ON (MATCH(t1.f1) AGAINST (""))
+ WHERE t1.f1 GROUP BY t1.f1))';
+
+EXECUTE stmt;
+EXECUTE stmt;
+
+DEALLOCATE PREPARE stmt;
+
+PREPARE stmt FROM
+'EXPLAIN SELECT 1 FROM t1
+ WHERE 1 > ALL((SELECT 1 FROM t1 JOIN t1 a
+ ON (MATCH(t1.f1) AGAINST (""))
+ WHERE t1.f1 GROUP BY t1.f1))';
+
+EXECUTE stmt;
+EXECUTE stmt;
+
+DEALLOCATE PREPARE stmt;
+
+DROP TABLE t1;
+
 --echo End of 5.1 tests.

sql/item_subselect.cc

@@ -1906,21 +1906,26 @@ int subselect_single_select_engine::exec()
       DBUG_RETURN(join->error ? join->error : 1);
     }
     if (!select_lex->uncacheable && thd->lex->describe && 
-        !(join->select_options & SELECT_DESCRIBE) && 
-        join->need_tmp)
+        !(join->select_options & SELECT_DESCRIBE))
     {
       item->update_used_tables();
       if (item->const_item())
       {
+        /*
+          It's necessary to keep original JOIN table because
+          create_sort_index() function may overwrite original
+          JOIN_TAB::type and wrong optimization method can be
+          selected on re-execution.
+        */
+        select_lex->uncacheable|= UNCACHEABLE_EXPLAIN;
+        select_lex->master_unit()->uncacheable|= UNCACHEABLE_EXPLAIN;
         /*
           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())
+        if (join->need_tmp && join->init_save_join_tab())
           DBUG_RETURN(1);                        /* purecov: inspected */
       }
     }

sql/sql_select.cc

@@ -2490,6 +2490,13 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
 	{
 	  DBUG_RETURN(TRUE);
 	}
+        /*
+          Original join tabs might be overwritten at first
+          subselect execution. So we need to restore them.
+        */
+        Item_subselect *subselect= select_lex->master_unit()->item;
+        if (subselect && subselect->is_uncacheable() && join->reinit())
+          DBUG_RETURN(TRUE);
       }
       else
       {
@@ -8825,6 +8832,13 @@ simplify_joins(JOIN *join, List<TABLE_LIST> *join_list, COND *conds, bool top)
         that reject nulls => the outer join can be replaced by an inner join.
       */
       table->outer_join= 0;
+      /*
+        Update TABLE::maybe_null field as it could have
+        the value(maybe_null==TRUE) based on the assumption
+        that this join is outer(see setup_table_map() func).
+      */
+      if (table->table)
+        table->table->maybe_null= FALSE;
       if (table->on_expr)
       {
         /* Add on expression to the where condition. */