sql_union.cc, sql_select.cc:

  Code clean-up
sql_union.cc, union.test, union.result:
  A fix for a bug #978.
  This enables that NULL's can be entered into UNION's result set,
  although first SELECT columns are NOT NULL.
  This is also a start of fixing UNION's properly regarding type
  acceptance.
sql_select.cc:
  A commit for my second July SPRINT task

mysql-test/r/union.result

@@ -328,3 +328,16 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	const	PRIMARY	PRIMARY	4	const	1	
 2	UNION	t1	ref	b	b	5	const	1	Using where
 drop table t1,t2;
+create table t1 (   id int not null auto_increment, primary key (id)   ,user_name text );
+create table t2 (    id int not null auto_increment, primary key (id)   ,group_name text );
+create table t3 (    id int not null auto_increment, primary key (id)   ,user_id int   ,index user_idx (user_id)   ,foreign key (user_id) references users(id)   ,group_id int   ,index group_idx (group_id)   ,foreign key (group_id) references groups(id) );
+insert into t1 (user_name) values ('Tester');
+insert into t2 (group_name) values ('Group A');
+insert into t2 (group_name) values ('Group B');
+insert into t3 (user_id, group_id) values (1,1);
+select 1 'is_in_group', a.user_name, c.group_name, b.id from t1 a, t3 b, t2 c where a.id = b.user_id and b.group_id = c.id UNION  select 0 'is_in_group', a.user_name, c.group_name, null from t1 a, t2 c;
+is_in_group	user_name	group_name	id
+1	Tester	Group A	1
+0	Tester	Group A	NULL
+0	Tester	Group B	NULL
+drop table t1, t2, t3;

mysql-test/t/union.test

@@ -209,3 +209,12 @@ explain (select * from t1 where a=1) union (select * from t2 where a=1);
 explain (select * from t1 where a=1 and b=10) union (select t1.a,t2.a from t1,t2 where t1.a=t2.a);
 explain (select * from t1 where a=1) union (select * from t1 where b=1);
 drop table t1,t2;
+create table t1 (   id int not null auto_increment, primary key (id)   ,user_name text );
+create table t2 (    id int not null auto_increment, primary key (id)   ,group_name text );
+create table t3 (    id int not null auto_increment, primary key (id)   ,user_id int   ,index user_idx (user_id)   ,foreign key (user_id) references users(id)   ,group_id int   ,index group_idx (group_id)   ,foreign key (group_id) references groups(id) );
+insert into t1 (user_name) values ('Tester');
+insert into t2 (group_name) values ('Group A');
+insert into t2 (group_name) values ('Group B');
+insert into t3 (user_id, group_id) values (1,1);
+select 1 'is_in_group', a.user_name, c.group_name, b.id from t1 a, t3 b, t2 c where a.id = b.user_id and b.group_id = c.id UNION  select 0 'is_in_group', a.user_name, c.group_name, null from t1 a, t2 c;
+drop table t1, t2, t3;

sql/sql_union.cc

@@ -154,8 +154,6 @@ int st_select_lex_unit::prepare(THD *thd, select_result *sel_result,
       goto err;
     List_iterator<Item> it(select_cursor->item_list);	
     Item *item;
-    while((item=it++))
-      item->maybe_null=1;
     item_list= select_cursor->item_list;
     select_cursor->with_wild= 0;
     if (setup_ref_array(thd, &select_cursor->ref_pointer_array, 
@@ -166,6 +164,12 @@ int st_select_lex_unit::prepare(THD *thd, select_result *sel_result,
 		     item_list, 0, 0, 1))
       goto err;
     t_and_f= 1;
+    while((item=it++))
+    {
+      item->maybe_null=1;
+      if (item->type() == Item::FIELD_ITEM)
+	((class Item_field *)item)->field->table->maybe_null=1;
+    }
   }
 
   tmp_table_param.field_count=item_list.elements;
@@ -249,7 +253,6 @@ int st_select_lex_unit::prepare(THD *thd, select_result *sel_result,
 
 int st_select_lex_unit::exec()
 {
-  int do_print_slow= 0;
   SELECT_LEX_NODE *lex_select_save= thd->lex.current_select;
   SELECT_LEX *select_cursor=first_select_in_union();
   DBUG_ENTER("st_select_lex_unit::exec");
@@ -317,7 +320,6 @@ int st_select_lex_unit::exec()
 	thd->lex.current_select= lex_select_save;
 	DBUG_RETURN(res);
       }
-      do_print_slow|= select_cursor->options;
     }
   }
   optimized= 1;
@@ -360,12 +362,6 @@ int st_select_lex_unit::exec()
 	Mark for slow query log if any of the union parts didn't use
 	indexes efficiently
       */
-      select_cursor->options= ((select_cursor->options &
-				~(QUERY_NO_INDEX_USED |
-				  QUERY_NO_GOOD_INDEX_USED)) |
-			       do_print_slow &
-			       (QUERY_NO_INDEX_USED |
-				QUERY_NO_GOOD_INDEX_USED));
     }
   }
   thd->lex.current_select= lex_select_save;