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MDEV-9959: A serious MariaDB server performance bug

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If a derived table has SELECT DISTINCT, provide index statistics for it so that the join optimizer in the
upper select knows that ref access to the table will produce one row.
This commit is contained in:
Varun Gupta 2019-04-30 21:05:50 +05:30
parent 8c8bee0a56
commit cb9fa1a08b
10 changed files with 153 additions and 7 deletions
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8
mysql-test/main/cte_nonrecursive.result
View File

@ -244,7 +244,7 @@ with t as (select distinct a from t1 where b >= 'c')
select * from t as r1, t as r2 where r1.a=r2.a;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 8 Using where
1 PRIMARY <derived3> ref key0 key0 5 r1.a 2
1 PRIMARY <derived3> ref key0 key0 5 r1.a 1
3 DERIVED t1 ALL NULL NULL NULL NULL 8 Using where; Using temporary
2 DERIVED t1 ALL NULL NULL NULL NULL 8 Using where; Using temporary
explain
@ -253,7 +253,7 @@ select * from (select distinct a from t1 where b >= 'c') as r1,
where r1.a=r2.a;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 8 Using where
1 PRIMARY <derived3> ref key0 key0 5 r1.a 2
1 PRIMARY <derived3> ref key0 key0 5 r1.a 1
3 DERIVED t1 ALL NULL NULL NULL NULL 8 Using where; Using temporary
2 DERIVED t1 ALL NULL NULL NULL NULL 8 Using where; Using temporary
# two references to t specified by a query
@ -369,7 +369,7 @@ select c as a from t2 where c < 4)
select * from t2,t where t2.c=t.a;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t2 ALL NULL NULL NULL NULL 4 Using where
1 PRIMARY <derived2> ref key0 key0 5 test.t2.c 2
1 PRIMARY <derived2> ref key0 key0 5 test.t2.c 1
2 DERIVED t1 ALL NULL NULL NULL NULL 8 Using where
3 UNION t2 ALL NULL NULL NULL NULL 4 Using where
NULL UNION RESULT <union2,3> ALL NULL NULL NULL NULL NULL
@ -381,7 +381,7 @@ select c as a from t2 where c < 4) as t
where t2.c=t.a;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t2 ALL NULL NULL NULL NULL 4 Using where
1 PRIMARY <derived2> ref key0 key0 5 test.t2.c 2
1 PRIMARY <derived2> ref key0 key0 5 test.t2.c 1
2 DERIVED t1 ALL NULL NULL NULL NULL 8 Using where
3 UNION t2 ALL NULL NULL NULL NULL 4 Using where
NULL UNION RESULT <union2,3> ALL NULL NULL NULL NULL NULL

54
mysql-test/main/derived.result
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@ -1195,3 +1195,57 @@ drop table t1,t2,t3;
#
# End of 10.2 tests
#
#
# MDEV-9959: A serious MariaDB server performance bug
#
create table t1(a int);
insert into t1 values (1),(2),(3),(4),(5),(6);
create table t2(a int, b int,c int);
insert into t2(a,b,c) values (1,1,2),(2,2,3),(3,1,4),(4,2,2),(5,1,1),(6,2,5);
create table t3(a int, b int);
insert into t3(a,b) values (1,1),(2,2),(2,1),(1,2),(5,1),(9,2);
table "<derived2>" should have type=ref and rows=1
one select in derived table
with distinct
analyze select * from t1 , ((select distinct t2.a from t2 order by c))q where t1.a=q.a;
id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00 Using where
1 PRIMARY <derived2> ref key0 key0 5 test.t1.a 1 1.00 100.00 100.00
2 DERIVED t2 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00 Using temporary; Using filesort
analyze select * from t1 , ((select distinct t2.a, t2.b from t2 order by c))q where t1.a=q.a;
id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00 Using where
1 PRIMARY <derived2> ref key0 key0 5 test.t1.a 2 1.00 100.00 100.00
2 DERIVED t2 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00 Using temporary; Using filesort
# multiple selects in derived table
# NO UNION ALL
analyze select * from t1 , ( (select t2.a from t2 order by c) union (select t2.a from t2 order by c))q where t1.a=q.a;
id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00 Using where
1 PRIMARY <derived2> ref key0 key0 5 test.t1.a 1 1.00 100.00 100.00
2 DERIVED t2 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00
3 UNION t2 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00
NULL UNION RESULT <union2,3> ALL NULL NULL NULL NULL NULL 6.00 NULL NULL
select * from t1 , ( (select t2.a from t2 order by c) union (select t2.a from t2 order by c))q where t1.a=q.a;
a a
1 1
2 2
3 3
4 4
5 5
6 6
# UNION ALL and EXCEPT
analyze select * from t1 , ( (select t2.a from t2 order by c) union all (select t2.a from t2 order by c) except(select t3.a from t3 order by b))q where t1.a=q.a;
id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00 Using where
1 PRIMARY <derived2> ref key0 key0 5 test.t1.a 1 0.50 100.00 100.00
2 DERIVED t2 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00
3 UNION t2 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00
4 EXCEPT t3 ALL NULL NULL NULL NULL 6 6.00 100.00 100.00
NULL UNIT RESULT <unit2,3,4> ALL NULL NULL NULL NULL NULL 3.00 NULL NULL
select * from t1 , ( (select t2.a from t2 order by c) union all (select t2.a from t2 order by c) except(select t3.a from t3 order by b))q where t1.a=q.a;
a a
3 3
4 4
6 6
drop table t1,t2,t3;

30
mysql-test/main/derived.test
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@ -1032,3 +1032,33 @@ drop table t1,t2,t3;
--echo #
--echo # End of 10.2 tests
--echo #
--echo #
--echo # MDEV-9959: A serious MariaDB server performance bug
--echo #
create table t1(a int);
insert into t1 values (1),(2),(3),(4),(5),(6);
create table t2(a int, b int,c int);
insert into t2(a,b,c) values (1,1,2),(2,2,3),(3,1,4),(4,2,2),(5,1,1),(6,2,5);
create table t3(a int, b int);
insert into t3(a,b) values (1,1),(2,2),(2,1),(1,2),(5,1),(9,2);
--echo table "<derived2>" should have type=ref and rows=1
--echo one select in derived table
--echo with distinct
analyze select * from t1 , ((select distinct t2.a from t2 order by c))q where t1.a=q.a;
analyze select * from t1 , ((select distinct t2.a, t2.b from t2 order by c))q where t1.a=q.a;
--echo # multiple selects in derived table
--echo # NO UNION ALL
analyze select * from t1 , ( (select t2.a from t2 order by c) union (select t2.a from t2 order by c))q where t1.a=q.a;
select * from t1 , ( (select t2.a from t2 order by c) union (select t2.a from t2 order by c))q where t1.a=q.a;
--echo # UNION ALL and EXCEPT
analyze select * from t1 , ( (select t2.a from t2 order by c) union all (select t2.a from t2 order by c) except(select t3.a from t3 order by b))q where t1.a=q.a;
select * from t1 , ( (select t2.a from t2 order by c) union all (select t2.a from t2 order by c) except(select t3.a from t3 order by b))q where t1.a=q.a;
drop table t1,t2,t3;

2
mysql-test/main/derived_view.result
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@ -1525,7 +1525,7 @@ EXPLAIN
SELECT a FROM t1 WHERE (a,b) IN (SELECT * FROM v2);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 2 Using where
1 PRIMARY <derived3> ref key0 key0 10 test.t1.a,test.t1.b 2 FirstMatch(t1)
1 PRIMARY <derived3> ref key0 key0 10 test.t1.a,test.t1.b 1 FirstMatch(t1)
3 DERIVED t2 ALL NULL NULL NULL NULL 6
4 UNION t3 ALL NULL NULL NULL NULL 4
NULL UNION RESULT <union3,4> ALL NULL NULL NULL NULL NULL

2
mysql-test/main/subselect_extra.result
View File

@ -409,7 +409,7 @@ EXPLAIN
SELECT a FROM t1 WHERE (a,b) IN (SELECT * FROM v2);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 2 Using where
1 PRIMARY <derived3> ref key0 key0 10 test.t1.a,test.t1.b 2 FirstMatch(t1)
1 PRIMARY <derived3> ref key0 key0 10 test.t1.a,test.t1.b 1 FirstMatch(t1)
3 DERIVED t2 ALL NULL NULL NULL NULL 6
4 UNION t3 ALL NULL NULL NULL NULL 4
NULL UNION RESULT <union3,4> ALL NULL NULL NULL NULL NULL

2
mysql-test/main/subselect_extra_no_semijoin.result
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@ -411,7 +411,7 @@ EXPLAIN
SELECT a FROM t1 WHERE (a,b) IN (SELECT * FROM v2);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 2 Using where
2 DEPENDENT SUBQUERY <derived3> index_subquery key0 key0 10 func,func 2 Using where
2 DEPENDENT SUBQUERY <derived3> index_subquery key0 key0 10 func,func 1 Using where
3 DERIVED t2 ALL NULL NULL NULL NULL 6
4 UNION t3 ALL NULL NULL NULL NULL 4
NULL UNION RESULT <union3,4> ALL NULL NULL NULL NULL NULL

1
sql/sql_class.cc
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@ -847,6 +847,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier, bool skip_global_sys_var_lock)
invoker.init();
prepare_derived_at_open= FALSE;
create_tmp_table_for_derived= FALSE;
force_read_stats= FALSE;
save_prep_leaf_list= FALSE;
org_charset= 0;
/* Restore THR_THD */

1
sql/sql_lex.h
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@ -896,6 +896,7 @@ public:
bool union_needs_tmp_table();
void set_unique_exclude();
bool check_distinct_in_union();
friend struct LEX;
friend int subselect_union_engine::exec();

40
sql/sql_union.cc
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@ -2049,3 +2049,43 @@ void st_select_lex_unit::set_unique_exclude()
}
}
}
/**
@brief
Check if the derived table is guaranteed to have distinct rows because of
UNION operations used to populate it.
@detail
UNION operation removes duplicate rows from its output. That is, a query like
select * from t1 UNION select * from t2
will not produce duplicate rows in its output, even if table t1 (and/or t2)
contain duplicate rows. EXCEPT and INTERSECT operations also have this
property.
On the other hand, UNION ALL operation doesn't remove duplicates. (The SQL
standard also defines EXCEPT ALL and INTERSECT ALL, but we don't support
them).
st_select_lex_unit computes its value left to right. That is, if there is
a st_select_lex_unit object describing
(select #1) OP1 (select #2) OP2 (select #3)
then ((select #1) OP1 (select #2)) is computed first, and OP2 is computed
second.
How can one tell if st_select_lex_unit is guaranteed to have distinct
output rows? This depends on whether the last operation was duplicate-
removing or not:
- UNION ALL is not duplicate-removing
- all other operations are duplicate-removing
*/
bool st_select_lex_unit::check_distinct_in_union()
{
if (union_distinct && !union_distinct->next_select())
return true;
return false;
}

20
sql/table.cc
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@ -7270,6 +7270,26 @@ bool TABLE::add_tmp_key(uint key, uint key_parts,
key_part_info++;
}
/*
For the case when there is a derived table that would give distinct rows,
the index statistics are passed to the join optimizer to tell that a ref
access to all the fields of the derived table will produce only one row.
*/
st_select_lex_unit* derived= pos_in_table_list ?
pos_in_table_list->derived: NULL;
if (derived)
{
st_select_lex* first= derived->first_select();
uint select_list_items= first->get_item_list()->elements;
if (key_parts == select_list_items)
{
if ((!first->is_part_of_union() && (first->options & SELECT_DISTINCT)) ||
derived->check_distinct_in_union())
keyinfo->rec_per_key[key_parts - 1]= 1;
}
}
set_if_bigger(s->max_key_length, keyinfo->key_length);
s->keys++;
return FALSE;