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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.
parent b1a6ecd6
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Showing with 222 additions and 16 deletions
mysql-test/r/group_min_max.result
View file @ 5c4e64a5
......@@ -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
View file @ 5c4e64a5
......@@ -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
View file @ 5c4e64a5
......@@ -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
......
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