MWL#89

Fixed query plans with loose index scan degraded into index scan.

Analysis:
With MWL#89 subqueries are no longer executed and substituted during
the optimization of the outer query. As a result subquery predicates
that were previously executed and substituted by a constant before
the range optimizer were present as regular subquery predicates during
range optimization. The procedure check_group_min_max_predicates()
had a naive test that ruled out all queries with subqueries in the
WHERE clause. This resulted in worse plans with MWL#89.

Solution:
The solution is to refine the test in check_group_min_max_predicates()
to check if each MIN/MAX argument is referred to by a subquery predicate.

mysql-test/r/group_min_max.result

@@ -1360,12 +1360,158 @@ group by a1,a2,b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	index	NULL	idx_t1_1	163	NULL	128	Using where; Using index
 2	DEPENDENT SUBQUERY	t2	index	NULL	idx_t2_1	163	NULL	164	Using where; Using index
+select a1,a2,b,min(c),max(c) from t1
+where exists ( select * from t2 where t2.c = t1.c )
+group by a1,a2,b;
+a1	a2	b	min(c)	max(c)
+a	a	a	a111	d111
+a	a	b	e112	h112
+a	b	a	i121	l121
+a	b	b	m122	p122
+b	a	a	a211	d211
+b	a	b	e212	h212
+b	b	a	i221	l221
+b	b	b	m222	p222
+c	a	a	a311	d311
+c	a	b	e312	h312
+c	b	a	i321	l321
+c	b	b	m322	p322
+d	a	a	a411	d411
+d	a	b	e412	h412
+d	b	a	i421	l421
+d	b	b	m422	p422
 explain select a1,a2,b,min(c),max(c) from t1
 where exists ( select * from t2 where t2.c > 'b1' )
 group by a1,a2,b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	t1	range	NULL	idx_t1_1	147	NULL	17	Using index for group-by
+1	PRIMARY	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by
 2	SUBQUERY	t2	index	NULL	idx_t2_1	163	NULL	164	Using where; Using index
+select a1,a2,b,min(c),max(c) from t1
+where exists ( select * from t2 where t2.c > 'b1' )
+group by a1,a2,b;
+a1	a2	b	min(c)	max(c)
+a	a	a	a111	d111
+a	a	b	e112	h112
+a	b	a	i121	l121
+a	b	b	m122	p122
+b	a	a	a211	d211
+b	a	b	e212	h212
+b	b	a	i221	l221
+b	b	b	m222	p222
+c	a	a	a311	d311
+c	a	b	e312	h312
+c	b	a	i321	l321
+c	b	b	m322	p322
+d	a	a	a411	d411
+d	a	b	e412	h412
+d	b	a	i421	l421
+d	b	b	m422	p422
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by
+2	DEPENDENT SUBQUERY	t2	index	NULL	idx_t2_1	163	NULL	164	Using where; Using index
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+a1	a2	b	c	min(c)	max(c)
+a	a	b	h112	e112	h112
+a	b	b	p122	m122	p122
+b	a	b	h212	e212	h212
+b	b	b	p222	m222	p222
+c	a	b	h312	e312	h312
+c	b	b	p322	m322	p322
+d	a	b	h412	e412	h412
+d	b	b	p422	m422	p422
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+where t2.c in (select c from t3 where t3.c > t1.b) and
+t2.c > 'b1' )
+group by a1,a2,b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by
+2	DEPENDENT SUBQUERY	t2	index	NULL	idx_t2_1	163	NULL	164	Using where; Using index
+2	DEPENDENT SUBQUERY	t3	index	NULL	idx_t3_1	10	NULL	192	Using where; Using index; FirstMatch(t2)
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+where t2.c in (select c from t3 where t3.c > t1.b) and
+t2.c > 'b1' )
+group by a1,a2,b;
+a1	a2	b	c	min(c)	max(c)
+a	a	a	d111	a111	d111
+a	a	b	h112	e112	h112
+a	b	a	l121	i121	l121
+a	b	b	p122	m122	p122
+b	a	a	d211	a211	d211
+b	a	b	h212	e212	h212
+b	b	a	l221	i221	l221
+b	b	b	p222	m222	p222
+c	a	a	d311	a311	d311
+c	a	b	h312	e312	h312
+c	b	a	l321	i321	l321
+c	b	b	p322	m322	p322
+d	a	a	d411	a411	d411
+d	a	b	h412	e412	h412
+d	b	a	l421	i421	l421
+d	b	b	p422	m422	p422
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	index	NULL	idx_t1_1	163	NULL	128	Using where; Using index
+2	DEPENDENT SUBQUERY	t2	index	NULL	idx_t2_1	163	NULL	164	Using where; Using index
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+a1	a2	b	c	min(c)	max(c)
+a	a	a	a111	a111	d111
+a	a	b	e112	e112	h112
+a	b	a	i121	i121	l121
+a	b	b	m122	m122	p122
+b	a	a	a211	a211	d211
+b	a	b	e212	e212	h212
+b	b	a	i221	i221	l221
+b	b	b	m222	m222	p222
+c	a	a	a311	a311	d311
+c	a	b	e312	e312	h312
+c	b	a	i321	i321	l321
+c	b	b	m322	m322	p322
+d	a	a	a411	a411	d411
+d	a	b	e412	e412	h412
+d	b	a	i421	i421	l421
+d	b	b	m422	m422	p422
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+where t2.c in (select c from t3 where t3.c > t1.c) and
+t2.c > 'b1' )
+group by a1,a2,b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	index	NULL	idx_t1_1	163	NULL	128	Using where; Using index
+2	DEPENDENT SUBQUERY	t2	index	NULL	idx_t2_1	163	NULL	164	Using where; Using index
+2	DEPENDENT SUBQUERY	t3	index	NULL	idx_t3_1	10	NULL	192	Using where; Using index; FirstMatch(t2)
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+where t2.c in (select c from t3 where t3.c > t1.c) and
+t2.c > 'b1' )
+group by a1,a2,b;
+a1	a2	b	c	min(c)	max(c)
+a	a	a	a111	a111	d111
+a	a	b	e112	e112	h112
+a	b	a	i121	i121	l121
+a	b	b	m122	m122	p122
+b	a	a	a211	a211	d211
+b	a	b	e212	e212	h212
+b	b	a	i221	i221	l221
+b	b	b	m222	m222	p222
+c	a	a	a311	a311	d311
+c	a	b	e312	e312	h312
+c	b	a	i321	i321	l321
+c	b	b	m322	m322	o322
+d	a	a	a411	a411	d411
+d	a	b	e412	e412	h412
+d	b	a	i421	i421	l421
+d	b	b	m422	m422	o422
 explain select a1,a2,b,min(c),max(c) from t1 where (a1 >= 'c' or a2 < 'b') and (b > 'a') group by a1,a2,b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	idx_t1_0,idx_t1_1,idx_t1_2	idx_t1_1	147	NULL	17	Using where; Using index for group-by
@@ -2246,11 +2392,11 @@ EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE EXISTS
 (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1_outer	index	NULL	a	10	NULL	15	Using index
-2	SUBQUERY	t1	index	NULL	a	10	NULL	15	Using index
+2	SUBQUERY	t1	index	NULL	a	10	NULL	1	Using index
 EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE 
 (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+1	PRIMARY	t1_outer	index	NULL	a	10	NULL	15	Using index
 2	SUBQUERY	t1	range	NULL	a	5	NULL	8	Using index for group-by
 EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE 
 a IN (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2);

mysql-test/t/group_min_max.test

@@ -406,11 +406,61 @@ explain select a1,a2,b,min(c),max(c) from t1
 where exists ( select * from t2 where t2.c = t1.c )
 group by a1,a2,b;
 
+select a1,a2,b,min(c),max(c) from t1
+where exists ( select * from t2 where t2.c = t1.c )
+group by a1,a2,b;
+
 # the sub-select is unrelated to MIN/MAX
 explain select a1,a2,b,min(c),max(c) from t1
 where exists ( select * from t2 where t2.c > 'b1' )
 group by a1,a2,b;
 
+select a1,a2,b,min(c),max(c) from t1
+where exists ( select * from t2 where t2.c > 'b1' )
+group by a1,a2,b;
+
+# correlated subselect that doesn't reference the min/max argument
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+               where t2.c in (select c from t3 where t3.c > t1.b) and
+               t2.c > 'b1' )
+group by a1,a2,b;
+
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+               where t2.c in (select c from t3 where t3.c > t1.b) and
+               t2.c > 'b1' )
+group by a1,a2,b;
+
+# correlated subselect that references the min/max argument
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
+group by a1,a2,b;
+
+explain select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+               where t2.c in (select c from t3 where t3.c > t1.c) and
+               t2.c > 'b1' )
+group by a1,a2,b;
+
+select a1,a2,b,c,min(c), max(c) from t1
+where exists ( select * from t2
+               where t2.c in (select c from t3 where t3.c > t1.c) and
+               t2.c > 'b1' )
+group by a1,a2,b;
+
 
 # A,B,C) Predicates referencing mixed classes of attributes
 # plans

sql/opt_range.cc

@@ -11102,7 +11102,7 @@ static bool get_constant_key_infix(KEY *index_info, SEL_ARG *index_range_tree,
                        uchar *key_infix, uint *key_infix_len,
                        KEY_PART_INFO **first_non_infix_part);
 static bool
-check_group_min_max_predicates(COND *cond, Item_field *min_max_arg_item,
+check_group_min_max_predicates(Item *cond, Item_field *min_max_arg_item,
                                Field::imagetype image_type);
 
 static void
@@ -11676,13 +11676,16 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree)
 */
 
 static bool
-check_group_min_max_predicates(COND *cond, Item_field *min_max_arg_item,
+check_group_min_max_predicates(Item *cond, Item_field *min_max_arg_item,
                                Field::imagetype image_type)
 {
   DBUG_ENTER("check_group_min_max_predicates");
   DBUG_ASSERT(cond && min_max_arg_item);
 
   cond= cond->real_item();
+  if (cond->get_cached_item())
+    cond= cond->get_cached_item();
+
   Item::Type cond_type= cond->type();
   if (cond_type == Item::COND_ITEM) /* 'AND' or 'OR' */
   {
@@ -11699,18 +11702,25 @@ check_group_min_max_predicates(COND *cond, Item_field *min_max_arg_item,
   }
 
   /*
-    TODO:
-    This is a very crude fix to handle sub-selects in the WHERE clause
-    (Item_subselect objects). With the test below we rule out from the
-    optimization all queries with subselects in the WHERE clause. What has to
-    be done, is that here we should analyze whether the subselect references
-    the MIN/MAX argument field, and disallow the optimization only if this is
-    so.
+    Disallow loose index scan if the MIN/MAX argument field is referenced by
+    a subquery in the WHERE clause.
   */
-  if (cond_type == Item::SUBSELECT_ITEM ||
-      (cond->get_cached_item() &&
-       cond->get_cached_item()->type() == Item::SUBSELECT_ITEM))
-    DBUG_RETURN(FALSE);
+  if (cond_type == Item::SUBSELECT_ITEM)
+  {
+    Item_subselect *subs_cond= (Item_subselect*) cond;
+    if (subs_cond->is_correlated)
+    {
+      DBUG_ASSERT(subs_cond->depends_on.elements > 0);
+      List_iterator_fast<Item*> li(subs_cond->depends_on);
+      Item **dep;
+      while ((dep= li++))
+      {
+        if ((*dep)->eq(min_max_arg_item, FALSE))
+          DBUG_RETURN(FALSE);
+      }
+    }
+    DBUG_RETURN(TRUE);
+  }
 
   /*
     Condition of the form 'field' is equivalent to 'field <> 0' and thus