Bug#32848: Data type conversion bug in union subselects in MySQL 5.0.38

There were two problems when inferring the correct field types resulting from
UNION queries.
- If the type is NULL for all corresponding fields in the UNION, the resulting 
  type would be NULL, while the type is BINARY(0) if there is just a single 
  SELECT NULL.
- If one SELECT in the UNION uses a subselect, a temporary table is created
  to represent the subselect, and the result type defaults to a STRING type,
  hiding the fact that the type was unknown(just a NULL value).
Fixed by remembering whenever a field was created from a NULL value and pass
type NULL to the type coercion if that is the case, and creating a string field
as result of UNION only if the type would otherwise be NULL.

mysql-test/r/union.result

@@ -1389,4 +1389,28 @@ select @var;
 1
 (select 2) union (select 1 into @var);
 ERROR 42000: Result consisted of more than one row
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1), (2), (3);
+CREATE TABLE t2 SELECT * FROM (SELECT NULL) a UNION SELECT a FROM t1;
+DESC t2;
+Field	Type	Null	Key	Default	Extra
+NULL	int(11)	YES		NULL	
+CREATE TABLE t3 SELECT a FROM t1 UNION SELECT * FROM (SELECT NULL) a;
+DESC t3;
+Field	Type	Null	Key	Default	Extra
+a	int(11)	YES		NULL	
+CREATE TABLE t4 SELECT NULL;
+DESC t4;
+Field	Type	Null	Key	Default	Extra
+NULL	binary(0)	YES		NULL	
+CREATE TABLE t5 SELECT NULL UNION SELECT NULL;
+DESC t5;
+Field	Type	Null	Key	Default	Extra
+NULL	binary(0)	YES		NULL	
+CREATE TABLE t6 
+SELECT * FROM (SELECT * FROM (SELECT NULL)a) b UNION SELECT a FROM t1;
+DESC t6;
+Field	Type	Null	Key	Default	Extra
+NULL	int(11)	YES		NULL	
+DROP TABLE t1, t2, t3, t4, t5, t6;
 End of 5.0 tests

mysql-test/t/union.test

@@ -877,4 +877,28 @@ DROP TABLE t1;
 select @var;
 --error 1172
 (select 2) union (select 1 into @var);
+
+#
+# Bug#32848: Data type conversion bug in union subselects in MySQL 5.0.38
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1), (2), (3);
+
+CREATE TABLE t2 SELECT * FROM (SELECT NULL) a UNION SELECT a FROM t1;
+DESC t2;
+
+CREATE TABLE t3 SELECT a FROM t1 UNION SELECT * FROM (SELECT NULL) a;
+DESC t3;
+
+CREATE TABLE t4 SELECT NULL;
+DESC t4;
+
+CREATE TABLE t5 SELECT NULL UNION SELECT NULL;
+DESC t5;
+
+CREATE TABLE t6 
+SELECT * FROM (SELECT * FROM (SELECT NULL)a) b UNION SELECT a FROM t1;
+DESC t6;
+
+DROP TABLE t1, t2, t3, t4, t5, t6;
 --echo End of 5.0 tests

sql/field.cc

@@ -1304,7 +1304,8 @@ Field::Field(char *ptr_arg,uint32 length_arg,uchar *null_ptr_arg,
    field_name(field_name_arg),
    query_id(0), key_start(0), part_of_key(0), part_of_sortkey(0),
    unireg_check(unireg_check_arg),
-   field_length(length_arg), null_bit(null_bit_arg) 
+   field_length(length_arg), null_bit(null_bit_arg), 
+   is_created_from_null_item(FALSE)
 {
   flags=null_ptr ? 0: NOT_NULL_FLAG;
   comment.str= (char*) "";

sql/field.h

@@ -88,6 +88,16 @@ public:
   uint          field_index;            // field number in fields array
   uint16	flags;
   uchar		null_bit;		// Bit used to test null bit
+  /**
+     If true, this field was created in create_tmp_field_from_item from a NULL
+     value. This means that the type of the field is just a guess, and the type
+     may be freely coerced to another type.
+
+     @see create_tmp_field_from_item
+     @see Item_type_holder::get_real_type
+
+   */
+  bool is_created_from_null_item;
 
   Field(char *ptr_arg,uint32 length_arg,uchar *null_ptr_arg,uchar null_bit_arg,
 	utype unireg_check_arg, const char *field_name_arg,

sql/item.cc

@@ -6569,6 +6569,8 @@ enum_field_types Item_type_holder::get_real_type(Item *item)
     */
     Field *field= ((Item_field *) item)->field;
     enum_field_types type= field->real_type();
+    if (field->is_created_from_null_item)
+      return MYSQL_TYPE_NULL;
     /* work around about varchar type field detection */
     if (type == MYSQL_TYPE_STRING && field->type() == MYSQL_TYPE_VAR_STRING)
       return MYSQL_TYPE_VAR_STRING;
@@ -6820,6 +6822,8 @@ Field *Item_type_holder::make_field_by_type(TABLE *table)
                          Field::NONE, name,
                          table, get_set_pack_length(enum_set_typelib->count),
                          enum_set_typelib, collation.collation);
+  case MYSQL_TYPE_NULL:
+    return make_string_field(table);
   default:
     break;
   }

sql/sql_select.cc

@@ -8974,6 +8974,8 @@ static Field *create_tmp_field_from_item(THD *thd, Item *item, TABLE *table,
     *((*copy_func)++) = item;			// Save for copy_funcs
   if (modify_item)
     item->set_result_field(new_field);
+  if (item->type() == MYSQL_TYPE_NULL)
+    new_field->is_created_from_null_item= TRUE;
   return new_field;
 }