database/mariadb
1
0
Fork 0
mirror of https://github.com/MariaDB/server.git synced 2025-08-01 03:47:19 +03:00
Code Activity

MDEV-20519: Query plan regression with optimizer_use_condition_selectivity > 1

Browse Source 
The issue here is the wrong estimate of the cardinality of a partial join,
the cardinality is too high because the function table_cond_selectivity()
returns an absurd number 100 while selectivity cannot be greater than 1.

When accessing table t by outer reference t1.a via index we do not perform any
range analysis for t. Yet we see TABLE::quick_key_parts[key] and
TABLE->quick_rows[key] contain a non-zero value though these should have been
remained untouched and equal to 0.

Thus real cause of the problem is that TABLE::init does not clean the arrays
TABLE::quick_key_parts[] and TABLE::>quick_rows[].
It should have done it because the TABLE structure created for any
instance of a table can be reused for many queries.
This commit is contained in:
Varun Gupta
2019-11-07 15:24:21 +05:30
parent 4e99e67c4e
commit b1ab2ba583
6 changed files with 166 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

56
mysql-test/r/selectivity_innodb.result
View File

@ -1820,6 +1820,62 @@ b a a b
9 9 10 10
set optimizer_use_condition_selectivity= @@optimizer_use_condition_selectivity;
drop table t1,t2,t3;
#
# MDEV-20519: Query plan regression with optimizer_use_condition_selectivity=4
#
create table t1 (id int, a int, PRIMARY KEY(id), key(a));
insert into t1 select seq,seq from seq_1_to_100;
create table t2 (id int, a int, b int, PRIMARY KEY(id), key(a), key(b));
insert into t2 select seq,seq,seq from seq_1_to_100;
set optimizer_switch='exists_to_in=off';
set optimizer_use_condition_selectivity=2;
SELECT * FROM t1
WHERE
EXISTS (SELECT * FROM t1 A INNER JOIN t2 ON t2.a = A.id
WHERE A.a=t1.a AND t2.b < 20);
id a
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 10
11 11
12 12
13 13
14 14
15 15
16 16
17 17
18 18
19 19
explain SELECT * FROM t1
WHERE
EXISTS (SELECT * FROM t1 A INNER JOIN t2 ON t2.a = A.id
WHERE A.a=t1.a AND t2.b < 20);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 index NULL a 5 NULL 100 Using where; Using index
2 DEPENDENT SUBQUERY A ref PRIMARY,a a 5 test.t1.a 1 Using index
2 DEPENDENT SUBQUERY t2 ref a,b a 5 test.A.id 1 Using where
EXPLAIN SELECT * FROM t1 A, t1 B WHERE A.a = B.a and A.id = 65;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE A const PRIMARY,a PRIMARY 4 const 1
1 SIMPLE B ref a a 5 const 1 Using index
explain SELECT * FROM t1
WHERE
EXISTS (SELECT * FROM t1 A INNER JOIN t2 ON t2.a = A.id
WHERE A.a=t1.a AND t2.b < 20);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 index NULL a 5 NULL 100 Using where; Using index
2 DEPENDENT SUBQUERY A ref PRIMARY,a a 5 test.t1.a 1 Using index
2 DEPENDENT SUBQUERY t2 ref a,b a 5 test.A.id 1 Using where
set optimizer_switch= @save_optimizer_switch;
set optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
drop table t1,t2;
# End of 10.1 tests
set optimizer_switch=@save_optimizer_switch_for_selectivity_test;
set @tmp_ust= @@use_stat_tables;