Bug #27786:

When merging views into the enclosing statement
the ORDER BY clause of the view is merged to the
parent's ORDER BY clause.
However when the VIEW is merged into an UNION
branch the ORDER BY should be ignored. 
Use of ORDER BY for individual SELECT statements
implies nothing about the order in which the rows
appear in the final result because UNION by default
produces unordered set of rows.
Fixed by ignoring the ORDER BY clause from the merge
view when expanded in an UNION branch.

mysql-test/r/view.result

@@ -3319,4 +3319,39 @@ lgid	clid
 2	YES
 DROP VIEW v1;
 DROP table t1,t2;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2),(3);
+CREATE VIEW v1 AS SELECT a FROM t1 ORDER BY a;
+SELECT * FROM t1 UNION SELECT * FROM v1;
+a
+1
+2
+3
+EXPLAIN SELECT * FROM t1 UNION SELECT * FROM v1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	3	
+2	UNION	t1	ALL	NULL	NULL	NULL	NULL	3	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	
+SELECT * FROM v1 UNION SELECT * FROM t1;
+a
+1
+2
+3
+EXPLAIN SELECT * FROM v1 UNION SELECT * FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	3	
+2	UNION	t1	ALL	NULL	NULL	NULL	NULL	3	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	
+SELECT * FROM t1 UNION SELECT * FROM v1 ORDER BY a;
+a
+1
+2
+3
+EXPLAIN SELECT * FROM t1 UNION SELECT * FROM v1 ORDER BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	3	
+2	UNION	t1	ALL	NULL	NULL	NULL	NULL	3	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
+DROP VIEW v1;
+DROP TABLE t1;
 End of 5.0 tests.

mysql-test/t/view.test

@@ -3205,4 +3205,20 @@ SELECT * FROM v1;
 DROP VIEW v1;
 DROP table t1,t2;
 
+#
+# Bug#27786: Inconsistent Operation Performing UNION On View With ORDER BY 
+#
+CREATE TABLE t1 (a INT); INSERT INTO t1 VALUES (1),(2),(3);
+CREATE VIEW v1 AS SELECT a FROM t1 ORDER BY a;
+
+SELECT * FROM t1 UNION SELECT * FROM v1;
+EXPLAIN SELECT * FROM t1 UNION SELECT * FROM v1;
+SELECT * FROM v1 UNION SELECT * FROM t1;
+EXPLAIN SELECT * FROM v1 UNION SELECT * FROM t1;
+SELECT * FROM t1 UNION SELECT * FROM v1 ORDER BY a;
+EXPLAIN SELECT * FROM t1 UNION SELECT * FROM v1 ORDER BY a;
+
+DROP VIEW v1;
+DROP TABLE t1;
+
 --echo End of 5.0 tests.

sql/sql_lex.h

@@ -468,6 +468,7 @@ public:
   bool change_result(select_subselect *result, select_subselect *old_result);
   void set_limit(st_select_lex *values);
   void set_thd(THD *thd_arg) { thd= thd_arg; }
+  inline bool is_union (); 
 
   friend void lex_start(THD *thd, uchar *buf, uint length);
   friend int subselect_union_engine::exec();
@@ -700,6 +701,13 @@ public:
 };
 typedef class st_select_lex SELECT_LEX;
 
+
+inline bool st_select_lex_unit::is_union ()
+{ 
+  return first_select()->next_select() && 
+    first_select()->next_select()->linkage == UNION_TYPE;
+}
+
 #define ALTER_ADD_COLUMN	1
 #define ALTER_DROP_COLUMN	2
 #define ALTER_CHANGE_COLUMN	4

sql/sql_view.cc

@@ -1263,13 +1263,18 @@ bool mysql_make_view(THD *thd, File_parser *parser, TABLE_LIST *table,
         unit->slave= save_slave; // fix include_down initialisation
       }
 
+      /* 
+        We can safely ignore the VIEW's ORDER BY if we merge into union 
+        branch, as order is not important there.
+      */
+      if (!table->select_lex->master_unit()->is_union())
+        table->select_lex->order_list.push_back(&lex->select_lex.order_list);
       /*
 	This SELECT_LEX will be linked in global SELECT_LEX list
 	to make it processed by mysql_handle_derived(),
 	but it will not be included to SELECT_LEX tree, because it
 	will not be executed
-      */
-      table->select_lex->order_list.push_back(&lex->select_lex.order_list);
+      */ 
       goto ok;
     }