From 57e5f8487f011b5ec7395acd75759d9061439f1b Mon Sep 17 00:00:00 2001
From: Gleb Shchepa <gshchepa@mysql.com>
Date: Sat, 6 Feb 2010 23:54:30 +0400
Subject: 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.
---
 sql/mysql_priv.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'sql/mysql_priv.h')

diff --git a/sql/mysql_priv.h b/sql/mysql_priv.h
index 34f29e7c458..1b775e658f1 100644
--- a/sql/mysql_priv.h
+++ b/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);
-- 
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