Fixed LP bug #944782.

This bug in the function JOIN::drop_unused_derived_keys() could
leave the internal structures for a materialized derived table
in an inconsistent state. This led to a not quite correct EXPLAIN
output when no additional key had been created to access the table.
It also may lead to more serious consequences: so, the test case
added with this fix caused a crash in mariadb-5.5.20.  

mysql-test/r/derived_view.result

@@ -420,7 +420,7 @@ join
 (select * from (select * from t1 where f1 < 7 group by f1) tt where f1 > 2) z
 on x.f1 = z.f1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	SIMPLE	<derived3>	ALL	key0	NULL	NULL	NULL	11	100.00	Using where
+1	SIMPLE	<derived3>	ALL	NULL	NULL	NULL	NULL	11	100.00	Using where
 1	SIMPLE	<derived5>	ref	key0	key0	5	tt.f1	2	100.00	
 5	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	11	100.00	Using where; Using temporary; Using filesort
 3	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	11	100.00	Using where; Using temporary; Using filesort
@@ -478,7 +478,7 @@ join
 (select * from t1 where f1 < 7 group by f1) tt where f1 > 2 group by f1) z
 on x.f1 = z.f1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	PRIMARY	<derived2>	ALL	key0	NULL	NULL	NULL	11	100.00	Using where
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	11	100.00	Using where
 1	PRIMARY	<derived4>	ref	key0	key0	5	x.f1	2	100.00	
 4	DERIVED	<derived5>	ALL	NULL	NULL	NULL	NULL	11	100.00	Using where; Using temporary; Using filesort
 5	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	11	100.00	Using where; Using temporary; Using filesort
@@ -1585,7 +1585,7 @@ a
 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	<derived2>	ALL	NULL	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
@@ -1929,5 +1929,37 @@ x
 y
 drop table t1,t2,t3;
 set SESSION optimizer_switch= @save_optimizer_switch;
+#
+# LP BUG#944782: derived table from an information schema table
+#
+SET @save_optimizer_switch=@@optimizer_switch;
+SET SESSION optimizer_switch='derived_merge=on';
+SET SESSION optimizer_switch='derived_with_keys=on';
+CREATE TABLE t1 (c1 int PRIMARY KEY, c2 char(5));
+EXPLAIN
+SELECT COUNT(*) > 0
+FROM INFORMATION_SCHEMA.COLUMNS
+INNER JOIN
+(SELECT TABLE_SCHEMA,
+GROUP_CONCAT(COLUMN_NAME ORDER BY SEQ_IN_INDEX ASC) AS COL_NAMES
+FROM INFORMATION_SCHEMA.STATISTICS
+GROUP BY TABLE_SCHEMA) AS UNIQUES
+ON ( COLUMNS.TABLE_SCHEMA = UNIQUES.TABLE_SCHEMA);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	COLUMNS	ALL	NULL	NULL	NULL	NULL	NULL	Open_frm_only; Scanned all databases
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	2	Using where; Using join buffer (flat, BNL join)
+2	DERIVED	STATISTICS	ALL	NULL	NULL	NULL	NULL	NULL	Open_frm_only; Scanned all databases; Using filesort
+SELECT COUNT(*) > 0
+FROM INFORMATION_SCHEMA.COLUMNS
+INNER JOIN
+(SELECT TABLE_SCHEMA,
+GROUP_CONCAT(COLUMN_NAME ORDER BY SEQ_IN_INDEX ASC) AS COL_NAMES
+FROM INFORMATION_SCHEMA.STATISTICS
+GROUP BY TABLE_SCHEMA) AS UNIQUES
+ON ( COLUMNS.TABLE_SCHEMA = UNIQUES.TABLE_SCHEMA);
+COUNT(*) > 0
+1
+DROP TABLE t1;
+set SESSION optimizer_switch= @save_optimizer_switch;
 set optimizer_switch=@exit_optimizer_switch;
 set join_cache_level=@exit_join_cache_level;

mysql-test/r/join_cache.result

@@ -5151,7 +5151,7 @@ EXPLAIN
 SELECT * FROM (SELECT DISTINCT * FROM t1) t
 WHERE t.a  IN (SELECT t2.a FROM t2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	<derived2>	ALL	key0	NULL	NULL	NULL	3	
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	3	
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	4	Using where; Start temporary; End temporary; Using join buffer (flat, BNL join)
 2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	3	Using temporary
 SELECT * FROM (SELECT DISTINCT * FROM t1) t

mysql-test/t/derived_view.test

@@ -1313,6 +1313,40 @@ drop table t1,t2,t3;
 
 set SESSION optimizer_switch= @save_optimizer_switch;
 
+--echo #
+--echo # LP BUG#944782: derived table from an information schema table
+--echo #
+
+SET @save_optimizer_switch=@@optimizer_switch;
+SET SESSION optimizer_switch='derived_merge=on'; 
+SET SESSION optimizer_switch='derived_with_keys=on'; 
+
+CREATE TABLE t1 (c1 int PRIMARY KEY, c2 char(5));
+
+EXPLAIN
+SELECT COUNT(*) > 0
+  FROM INFORMATION_SCHEMA.COLUMNS
+       INNER JOIN
+       (SELECT TABLE_SCHEMA,
+               GROUP_CONCAT(COLUMN_NAME ORDER BY SEQ_IN_INDEX ASC) AS COL_NAMES
+          FROM INFORMATION_SCHEMA.STATISTICS
+        GROUP BY TABLE_SCHEMA) AS UNIQUES
+        ON ( COLUMNS.TABLE_SCHEMA = UNIQUES.TABLE_SCHEMA);
+
+#  this query crashed in mariadb-5.5.20
+SELECT COUNT(*) > 0
+  FROM INFORMATION_SCHEMA.COLUMNS
+       INNER JOIN
+       (SELECT TABLE_SCHEMA,
+               GROUP_CONCAT(COLUMN_NAME ORDER BY SEQ_IN_INDEX ASC) AS COL_NAMES
+          FROM INFORMATION_SCHEMA.STATISTICS
+        GROUP BY TABLE_SCHEMA) AS UNIQUES
+        ON ( COLUMNS.TABLE_SCHEMA = UNIQUES.TABLE_SCHEMA);
+
+DROP TABLE t1;
+
+set SESSION optimizer_switch= @save_optimizer_switch;
+
 # The following command must be the last one the file 
 set optimizer_switch=@exit_optimizer_switch;
 set join_cache_level=@exit_join_cache_level;

sql/sql_select.cc

@@ -8712,8 +8712,13 @@ void JOIN::drop_unused_derived_keys()
       continue;
     if (table->max_keys > 1)
       table->use_index(tab->ref.key);
-    if (table->s->keys && tab->ref.key >= 0)
-      tab->ref.key= 0;
+    if (table->s->keys)
+    {
+      if (tab->ref.key >= 0)
+        tab->ref.key= 0;
+      else
+        table->s->keys= 0;
+    }
     tab->keys= (key_map) (table->s->keys ? 1 : 0);
   }
 }