fix for LP BUG#921878.

Problem was in try to check/use Item_direct_ref of derived view when we have to use real Item_field under it.

mysql-test/r/derived_view.result

@@ -1800,6 +1800,7 @@ INSERT INTO t2 VALUES (7), (4);
 CREATE TABLE t1 (b int NOT NULL);
 INSERT INTO t1 VALUES (5), (7);
 CREATE ALGORITHM=MERGE VIEW v1 AS SELECT * FROM t1;
+SET @save_optimizer_switch=@@optimizer_switch;
 SET SESSION optimizer_switch='derived_merge=off';
 PREPARE st1 FROM
 'SELECT * FROM (SELECT * FROM t2 LEFT JOIN v1 ON t2.a = v1.b) AS t';
@@ -1812,9 +1813,9 @@ a	b
 7	7
 4	NULL
 DEALLOCATE PREPARE st1;
+set SESSION optimizer_switch= @save_optimizer_switch;
 DROP VIEW v1;
 DROP TABLE t1,t2;
-SET SESSION optimizer_switch='derived_merge=on';
 #
 # LP bug #879939: assertion in ha_maria::enable_indexes 
 #                 with derived_with_keys=on
@@ -1832,6 +1833,7 @@ INSERT INTO t1 VALUES
 ('USA','Mesquite'), ('USA','Metairie'), ('USA','Miami');
 CREATE TABLE t3 (a varchar(35));
 INSERT INTO t3 VALUES ('Miami');
+SET @save_optimizer_switch=@@optimizer_switch;
 SET optimizer_switch = 'derived_with_keys=on';
 SET @@tmp_table_size=1024*4;
 explain SELECT * FROM (SELECT t1.* FROM t1, t2) AS t JOIN t3 ON t3.a = t.b;
@@ -1855,6 +1857,7 @@ USA	Miami	Miami
 USA	Miami	Miami
 USA	Miami	Miami
 SET @@tmp_table_size=default;
+set SESSION optimizer_switch= @save_optimizer_switch;
 drop table t1,t2,t3;
 #
 # BUG#882994: Crash in QUICK_RANGE_SELECT::reset with derived_with_keys
@@ -1906,5 +1909,25 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	3	
 2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	8	
 DROP TABLE t1;
+#
+# LP BUG#921878 incorrect check of items during columns union types
+# aggregation for merged derived tables
+#
+SET @save_optimizer_switch=@@optimizer_switch;
+SET SESSION optimizer_switch='derived_merge=on';
+CREATE TABLE t1 ( a ENUM( 'x', 'y' ) );
+insert into t1 values ('x');
+CREATE TABLE t2 LIKE t1;
+insert into t1 values ('y');
+CREATE TABLE t3 LIKE t1;
+INSERT INTO t3
+SELECT * FROM ( SELECT * FROM t1 ) AS A
+UNION SELECT * FROM t2;
+select * from t3;
+a
+x
+y
+drop table t1,t2,t3;
+set SESSION optimizer_switch= @save_optimizer_switch;
 set optimizer_switch=@exit_optimizer_switch;
 set join_cache_level=@exit_join_cache_level;

mysql-test/t/derived_view.test

@@ -1192,7 +1192,7 @@ INSERT INTO t2 VALUES (7), (4);
 CREATE TABLE t1 (b int NOT NULL);
 INSERT INTO t1 VALUES (5), (7);
 CREATE ALGORITHM=MERGE VIEW v1 AS SELECT * FROM t1;
-
+SET @save_optimizer_switch=@@optimizer_switch;
 SET SESSION optimizer_switch='derived_merge=off';
 
 PREPARE st1 FROM
@@ -1201,10 +1201,12 @@ EXECUTE st1;
 EXECUTE st1;
 DEALLOCATE PREPARE st1;
 
+set SESSION optimizer_switch= @save_optimizer_switch;
+
 DROP VIEW v1;
 DROP TABLE t1,t2;
 
-SET SESSION optimizer_switch='derived_merge=on';
+
 
 --echo #
 --echo # LP bug #879939: assertion in ha_maria::enable_indexes 
@@ -1226,7 +1228,7 @@ INSERT INTO t1 VALUES
 
 CREATE TABLE t3 (a varchar(35));
 INSERT INTO t3 VALUES ('Miami');
-
+SET @save_optimizer_switch=@@optimizer_switch;
 SET optimizer_switch = 'derived_with_keys=on';
 SET @@tmp_table_size=1024*4;
 explain SELECT * FROM (SELECT t1.* FROM t1, t2) AS t JOIN t3 ON t3.a = t.b;
@@ -1234,7 +1236,7 @@ SELECT * FROM (SELECT t1.* FROM t1, t2) AS t JOIN t3 ON t3.a = t.b;
 SET @@tmp_table_size=1024*1024*16;
 SELECT * FROM (SELECT t1.* FROM t1, t2) AS t JOIN t3 ON t3.a = t.b;
 SET @@tmp_table_size=default;
-
+set SESSION optimizer_switch= @save_optimizer_switch;
 drop table t1,t2,t3;
 
 --echo #
@@ -1291,6 +1293,26 @@ SELECT * FROM (SELECT * FROM t1 LIMIT 3) t;
 
 DROP TABLE t1;
 
+--echo #
+--echo # LP BUG#921878 incorrect check of items during columns union types
+--echo # aggregation for merged derived tables
+--echo #
+SET @save_optimizer_switch=@@optimizer_switch;
+SET SESSION optimizer_switch='derived_merge=on'; 
+CREATE TABLE t1 ( a ENUM( 'x', 'y' ) );
+insert into t1 values ('x');
+CREATE TABLE t2 LIKE t1;
+insert into t1 values ('y');
+CREATE TABLE t3 LIKE t1;
+INSERT INTO t3
+  SELECT * FROM ( SELECT * FROM t1 ) AS A
+    UNION SELECT * FROM t2;
+select * from t3;
+
+drop table t1,t2,t3;
+
+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/item.cc

@@ -9052,13 +9052,13 @@ void Item_type_holder::get_full_info(Item *item)
     DBUG_ASSERT((enum_set_typelib &&
                  get_real_type(item) == MYSQL_TYPE_NULL) ||
                 (!enum_set_typelib &&
-                 item->type() == Item::FIELD_ITEM &&
-                 (get_real_type(item) == MYSQL_TYPE_ENUM ||
-                  get_real_type(item) == MYSQL_TYPE_SET) &&
-                 ((Field_enum*)((Item_field *) item)->field)->typelib));
+                 item->real_item()->type() == Item::FIELD_ITEM &&
+                 (get_real_type(item->real_item()) == MYSQL_TYPE_ENUM ||
+                  get_real_type(item->real_item()) == MYSQL_TYPE_SET) &&
+                 ((Field_enum*)((Item_field *) item->real_item())->field)->typelib));
     if (!enum_set_typelib)
     {
-      enum_set_typelib= ((Field_enum*)((Item_field *) item)->field)->typelib;
+      enum_set_typelib= ((Field_enum*)((Item_field *) item->real_item())->field)->typelib;
     }
   }
 }