Fixed LP bug #878199.

The function JOIN::drop_unused_derived_keys could erroneously set
the value of REF::key to 0 for a joined materialized view/derived table
in the case when no REF access to the table was used by the query
execution plan. This could cause a crash of the server.

mysql-test/r/derived_view.result

@@ -1534,4 +1534,26 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
 DROP VIEW v1;
 DROP TABLE t1,t2;
+#
+# LP bug #878199: join of two materialized views
+#
+CREATE TABLE t1 (a int, b varchar(1)) ;
+INSERT INTO t1 VALUES (7,'c'), (3,'h'), (7,'c');
+CREATE TABLE t2 (b varchar(1)) ;
+INSERT INTO t2 VALUES ('p'), ('c'), ('j'), ('c'), ('p');
+CREATE VIEW v1 AS SELECT * FROM t1 GROUP BY a,b;
+CREATE VIEW v2 AS SELECT * FROM t2 GROUP BY b;
+SET SESSION optimizer_switch = 'derived_with_keys=on';
+SELECT v1.a FROM v1,v2 WHERE v2.b = v1.b ORDER BY 1;
+a
+7
+EXPLAIN
+SELECT v1.a FROM v1,v2 WHERE v2.b = v1.b ORDER BY 1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	ALL	key0	NULL	NULL	NULL	3	Using where; Using filesort
+1	PRIMARY	<derived3>	ref	key0	key0	5	v1.b	2	
+3	DERIVED	t2	ALL	NULL	NULL	NULL	NULL	5	Using temporary; Using filesort
+2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	3	Using temporary; Using filesort
+DROP VIEW v1,v2;
+DROP TABLE t1,t2;
 set optimizer_switch=@exit_optimizer_switch;

mysql-test/t/derived_view.test

@@ -973,5 +973,28 @@ SELECT v1.a FROM v1
 DROP VIEW v1;
 DROP TABLE t1,t2;
 
+--echo #
+--echo # LP bug #878199: join of two materialized views
+--echo #
+
+CREATE TABLE t1 (a int, b varchar(1)) ;
+INSERT INTO t1 VALUES (7,'c'), (3,'h'), (7,'c');
+
+CREATE TABLE t2 (b varchar(1)) ;
+INSERT INTO t2 VALUES ('p'), ('c'), ('j'), ('c'), ('p');
+
+CREATE VIEW v1 AS SELECT * FROM t1 GROUP BY a,b;
+
+CREATE VIEW v2 AS SELECT * FROM t2 GROUP BY b;
+
+SET SESSION optimizer_switch = 'derived_with_keys=on';
+
+SELECT v1.a FROM v1,v2 WHERE v2.b = v1.b ORDER BY 1;
+EXPLAIN
+SELECT v1.a FROM v1,v2 WHERE v2.b = v1.b ORDER BY 1;
+
+DROP VIEW v1,v2;
+DROP TABLE t1,t2;
+
 # The following command must be the last one the file 
 set optimizer_switch=@exit_optimizer_switch;

sql/sql_select.cc

@@ -8541,7 +8541,7 @@ void JOIN::drop_unused_derived_keys()
       continue;
     if (table->max_keys > 1)
       table->use_index(tab->ref.key);
-    if (table->s->keys)
+    if (table->s->keys && tab->ref.key >= 0)
       tab->ref.key= 0;
     tab->keys= (key_map) (table->s->keys ? 1 : 0);
   }