Bug #45640: optimizer bug produces wrong results

Grouping by a subquery in a query with a distinct aggregate
function lead to a wrong result (wrong and unordered
grouping values).

There are two related problems:

1) The query like this:

   SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) c
   FROM t1 GROUP BY aa

returned wrong result, because the outer reference "t1.a"
in the subquery was substituted with the Item_ref item.

The Item_ref item obtains data from the result_field object
that refreshes once after the end of each group. This data
is not applicable to filesort since filesort() doesn't care
about groups (and doesn't update result_field objects with
copy_fields() and so on). Also that data is not applicable
to group separation algorithm: end_send_group() checks every
record with test_if_group_changed() that evaluates Item_ref
items, but it refreshes those Item_ref-s only after the end
of group, that is a vicious circle and the grouped column
values in the output are shifted.

Fix: if
       a) we grouping by a subquery and
       b) that subquery has outer references to FROM list
          of the grouping query,
     then we substitute these outer references with
     Item_direct_ref like references under aggregate
     functions: Item_direct_ref obtains data directly
     from the current record.

2) The query with a non-trivial grouping expression like:

   SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) c
   FROM t1 GROUP BY aa+0

also returned wrong result, since JOIN::exec() substitutes
references to top-level aliases in SELECT list with Item_copy
caching items. Item_copy items have same refreshing policy
as Item_ref items, so the whole groping expression with
Item_copy inside returns wrong result in filesort() and
end_send_group().

Fix: include aliased items into GROUP BY item tree instead
     of Item_ref references to them.



mysql-test/r/group_by.result:
  Test case for bug #45640
mysql-test/t/group_by.test:
  Test case for bug #45640
sql/item.cc:
  Bug #45640: optimizer bug produces wrong results
  
  Item_field::fix_fields() has been modified to resolve
  aliases in GROUP BY item trees into aliased items instead
  of Item_ref items.
sql/item.h:
  Bug #45640: optimizer bug produces wrong results
  
  - Item::find_item_processor() has been introduced.
  - Item_ref::walk() has been modified to apply processors
    to itself too (not only to referenced item).
sql/mysql_priv.h:
  Bug #45640: optimizer bug produces wrong results
  
  fix_inner_refs() has been modified to accept group_list
  parameter.
sql/sql_lex.cc:
  Bug #45640: optimizer bug produces wrong results
  
  Initialization of st_select_lex::group_fix_field has
  been added.
sql/sql_lex.h:
  Bug #45640: optimizer bug produces wrong results
  
  The st_select_lex::group_fix_field field has been introduced
  to control alias resolution in Itef_fied::fix_fields.
sql/sql_select.cc:
  Bug #45640: optimizer bug produces wrong results
  
  - The fix_inner_refs function has been modified to treat
    subquery outer references like outer fields under aggregate
    functions, if they are included in GROUP BY item tree.
  
  - The find_order_in_list function has been modified to
    fix Item_field alias fields included in the GROUP BY item
    trees in a special manner.

mysql-test/r/group_by.result

@@ -1703,3 +1703,91 @@ COUNT(i)
 1
 DROP TABLE t1;
 SET @@sql_mode = @old_sql_mode;
+#
+# Bug #45640: optimizer bug produces wrong results
+#
+CREATE TABLE t1 (a INT, b INT);
+INSERT INTO t1 VALUES (4, 40), (1, 10), (2, 20), (2, 20), (3, 30);
+# should return 4 ordered records:
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa;
+aa	COUNT(DISTINCT b)
+1	1
+2	1
+3	1
+4	1
+SELECT (SELECT (SELECT t1.a)) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa;
+aa	COUNT(DISTINCT b)
+1	1
+2	1
+3	1
+4	1
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa+0;
+aa	COUNT(DISTINCT b)
+1	1
+2	1
+3	1
+4	1
+# should return the same result in a reverse order:
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY -aa;
+aa	COUNT(DISTINCT b)
+4	1
+3	1
+2	1
+1	1
+# execution plan should not use temporary table:
+EXPLAIN EXTENDED
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa+0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	5	100.00	Using filesort
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1276	Field or reference 'test.t1.a' of SELECT #2 was resolved in SELECT #1
+Note	1003	select (select `test`.`t1`.`a` AS `a`) AS `aa`,count(distinct `test`.`t1`.`b`) AS `COUNT(DISTINCT b)` from `test`.`t1` group by ((select `test`.`t1`.`a` AS `a`) + 0)
+EXPLAIN EXTENDED
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY -aa;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	5	100.00	Using filesort
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1276	Field or reference 'test.t1.a' of SELECT #2 was resolved in SELECT #1
+Note	1003	select (select `test`.`t1`.`a` AS `a`) AS `aa`,count(distinct `test`.`t1`.`b`) AS `COUNT(DISTINCT b)` from `test`.`t1` group by -((select `test`.`t1`.`a` AS `a`))
+# should return only one record
+SELECT (SELECT tt.a FROM t1 tt LIMIT 1) aa, COUNT(DISTINCT b) FROM t1
+GROUP BY aa;
+aa	COUNT(DISTINCT b)
+4	4
+CREATE TABLE t2 SELECT DISTINCT a FROM t1;
+# originally reported queries (1st two columns of next two query
+# results should be same):
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(DISTINCT b)
+FROM t1 GROUP BY aa, b;
+aa	b	COUNT(DISTINCT b)
+1	10	1
+2	20	1
+3	30	1
+4	40	1
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(         b)
+FROM t1 GROUP BY aa, b;
+aa	b	COUNT(         b)
+1	10	1
+2	20	2
+3	30	1
+4	40	1
+# ORDER BY for sure:
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(DISTINCT b)
+FROM t1 GROUP BY aa, b ORDER BY -aa, -b;
+aa	b	COUNT(DISTINCT b)
+4	40	1
+3	30	1
+2	20	1
+1	10	1
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(         b)
+FROM t1 GROUP BY aa, b ORDER BY -aa, -b;
+aa	b	COUNT(         b)
+4	40	1
+3	30	1
+2	20	2
+1	10	1
+DROP TABLE t1, t2;
+#
+# End of 5.1 tests

mysql-test/t/group_by.test

@@ -1158,3 +1158,53 @@ SELECT COUNT(i) FROM t1 WHERE i > 1;
 DROP TABLE t1;
 SET @@sql_mode = @old_sql_mode;
 
+--echo #
+--echo # Bug #45640: optimizer bug produces wrong results
+--echo #
+
+CREATE TABLE t1 (a INT, b INT);
+INSERT INTO t1 VALUES (4, 40), (1, 10), (2, 20), (2, 20), (3, 30);
+
+--echo # should return 4 ordered records:
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa;
+
+SELECT (SELECT (SELECT t1.a)) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa;
+
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa+0;
+
+--echo # should return the same result in a reverse order:
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY -aa;
+
+--echo # execution plan should not use temporary table:
+EXPLAIN EXTENDED
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY aa+0;
+
+EXPLAIN EXTENDED
+SELECT (SELECT t1.a) aa, COUNT(DISTINCT b) FROM t1 GROUP BY -aa;
+
+--echo # should return only one record
+SELECT (SELECT tt.a FROM t1 tt LIMIT 1) aa, COUNT(DISTINCT b) FROM t1
+  GROUP BY aa;
+
+CREATE TABLE t2 SELECT DISTINCT a FROM t1;
+
+--echo # originally reported queries (1st two columns of next two query
+--echo # results should be same):
+
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(DISTINCT b)
+  FROM t1 GROUP BY aa, b;
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(         b)
+  FROM t1 GROUP BY aa, b;
+
+--echo # ORDER BY for sure:
+
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(DISTINCT b)
+  FROM t1 GROUP BY aa, b ORDER BY -aa, -b;
+SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(         b)
+  FROM t1 GROUP BY aa, b ORDER BY -aa, -b;
+
+DROP TABLE t1, t2;
+
+--echo #
+
+--echo # End of 5.1 tests

sql/item.cc

@@ -4300,17 +4300,33 @@ bool Item_field::fix_fields(THD *thd, Item **reference)
               It's not an Item_field in the select list so we must make a new
               Item_ref to point to the Item in the select list and replace the
               Item_field created by the parser with the new Item_ref.
+
+              NOTE: If we are fixing an alias reference inside ORDER/GROUP BY
+              item tree, then we use new Item_ref as an intermediate value
+              to resolve referenced item only.
+              In this case the new Item_ref item is unused.
             */
             Item_ref *rf= new Item_ref(context, db_name,table_name,field_name);
             if (!rf)
               return 1;
-            thd->change_item_tree(reference, rf);
+
+            bool save_group_fix_field= thd->lex->current_select->group_fix_field;
             /*
-              Because Item_ref never substitutes itself with other items 
+              No need for recursive resolving of aliases.
-              in Item_ref::fix_fields(), we can safely use the original 
-              pointer to it even after fix_fields()
             */
-            return rf->fix_fields(thd, reference) ||  rf->check_cols(1);
+            thd->lex->current_select->group_fix_field= 0;
+
+            bool ret= rf->fix_fields(thd, (Item **) &rf) || rf->check_cols(1);
+            thd->lex->current_select->group_fix_field= save_group_fix_field;
+            if (ret)
+              return TRUE;
+
+            if (save_group_fix_field && alias_name_used)
+              thd->change_item_tree(reference, *rf->ref);
+            else
+              thd->change_item_tree(reference, rf);
+
+            return FALSE;
           }
         }
       }

sql/item.h

@@ -901,6 +901,7 @@ public:
   virtual bool change_context_processor(uchar *context) { return 0; }
   virtual bool reset_query_id_processor(uchar *query_id_arg) { return 0; }
   virtual bool is_expensive_processor(uchar *arg) { return 0; }
+  virtual bool find_item_processor(uchar *arg) { return this == (void *) arg; }
   virtual bool register_field_in_read_map(uchar *arg) { return 0; }
   /*
     Check if a partition function is allowed
@@ -2275,7 +2276,10 @@ public:
     return ref ? (*ref)->real_item() : this;
   }
   bool walk(Item_processor processor, bool walk_subquery, uchar *arg)
-  { return (*ref)->walk(processor, walk_subquery, arg); }
+  {
+    return (*ref)->walk(processor, walk_subquery, arg) ||
+           (this->*processor)(arg);
+  }
   virtual void print(String *str, enum_query_type query_type);
   bool result_as_longlong()
   {

sql/mysql_priv.h

@@ -1192,7 +1192,7 @@ int setup_group(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
 		List<Item> &fields, List<Item> &all_fields, ORDER *order,
 		bool *hidden_group_fields);
 bool fix_inner_refs(THD *thd, List<Item> &all_fields, SELECT_LEX *select,
-                   Item **ref_pointer_array);
+                   Item **ref_pointer_array, ORDER *group_list= NULL);
 
 bool handle_select(THD *thd, LEX *lex, select_result *result,
                    ulong setup_tables_done_option);

sql/sql_lex.cc

@@ -1592,6 +1592,7 @@ void st_select_lex::init_query()
   having= prep_having= where= prep_where= 0;
   olap= UNSPECIFIED_OLAP_TYPE;
   having_fix_field= 0;
+  group_fix_field= 0;
   context.select_lex= this;
   context.init();
   /*

sql/sql_lex.h

@@ -647,6 +647,8 @@ public:
   bool  braces;   	/* SELECT ... UNION (SELECT ... ) <- this braces */
   /* TRUE when having fix field called in processing of this SELECT */
   bool having_fix_field;
+  /* TRUE when GROUP BY fix field called in processing of this SELECT */
+  bool group_fix_field;
   /* List of references to fields referenced from inner selects */
   List<Item_outer_ref> inner_refs_list;
   /* Number of Item_sum-derived objects in this SELECT */

sql/sql_select.cc

@@ -287,6 +287,7 @@ bool handle_select(THD *thd, LEX *lex, select_result *result,
     all_fields        List of all fields used in select
     select            Current select
     ref_pointer_array Array of references to Items used in current select
+    group_list        GROUP BY list (is NULL by default)
 
   DESCRIPTION
     The function serves 3 purposes - adds fields referenced from inner
@@ -305,6 +306,8 @@ bool handle_select(THD *thd, LEX *lex, select_result *result,
       function is aggregated in the select where the outer field was
       resolved or in some more inner select then the Item_direct_ref
       class should be used.
+      Also it should be used if we are grouping by a subquery containing
+      the outer field.
     The resolution is done here and not at the fix_fields() stage as
     it can be done only after sum functions are fixed and pulled up to
     selects where they are have to be aggregated.
@@ -321,7 +324,7 @@ bool handle_select(THD *thd, LEX *lex, select_result *result,
 
 bool
 fix_inner_refs(THD *thd, List<Item> &all_fields, SELECT_LEX *select,
-                 Item **ref_pointer_array)
+                 Item **ref_pointer_array, ORDER *group_list)
 {
   Item_outer_ref *ref;
   bool res= FALSE;
@@ -371,6 +374,22 @@ fix_inner_refs(THD *thd, List<Item> &all_fields, SELECT_LEX *select,
         }
       }
     }
+    else
+    {
+      /*
+        Check if GROUP BY item trees contain the outer ref:
+        in this case we have to use Item_direct_ref instead of Item_ref.
+      */
+      for (ORDER *group= group_list; group; group= group->next)
+      {
+        if ((*group->item)->walk(&Item::find_item_processor, TRUE,
+                                 (uchar *) ref))
+        {
+          direct_ref= TRUE;
+          break;
+        }
+      }
+    }
     new_ref= direct_ref ?
               new Item_direct_ref(ref->context, item_ref, ref->table_name,
                           ref->field_name, ref->alias_name_used) :
@@ -577,7 +596,8 @@ JOIN::prepare(Item ***rref_pointer_array,
   }
 
   if (select_lex->inner_refs_list.elements &&
-      fix_inner_refs(thd, all_fields, select_lex, ref_pointer_array))
+      fix_inner_refs(thd, all_fields, select_lex, ref_pointer_array,
+                     group_list))
     DBUG_RETURN(-1);
 
   if (group_list)
@@ -14547,11 +14567,29 @@ find_order_in_list(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
 
     We check order_item->fixed because Item_func_group_concat can put
     arguments for which fix_fields already was called.
+    
+    group_fix_field= TRUE is to resolve aliases from the SELECT list
+    without creating of Item_ref-s: JOIN::exec() wraps aliased items
+    in SELECT list with Item_copy items. To re-evaluate such a tree
+    that includes Item_copy items we have to refresh Item_copy caches,
+    but:
+      - filesort() never refresh Item_copy items,
+      - end_send_group() checks every record for group boundary by the
+        test_if_group_changed function that obtain data from these
+        Item_copy items, but the copy_fields function that
+        refreshes Item copy items is called after group boundaries only -
+        that is a vicious circle.
+    So we prevent inclusion of Item_copy items.
   */
-  if (!order_item->fixed &&
+  bool save_group_fix_field= thd->lex->current_select->group_fix_field;
+  if (is_group_field)
+    thd->lex->current_select->group_fix_field= TRUE;
+  bool ret= (!order_item->fixed &&
       (order_item->fix_fields(thd, order->item) ||
        (order_item= *order->item)->check_cols(1) ||
-       thd->is_fatal_error))
+       thd->is_fatal_error));
+  thd->lex->current_select->group_fix_field= save_group_fix_field;
+  if (ret)
     return TRUE; /* Wrong field. */
 
   uint el= all_fields.elements;