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MDEV-36169: Two subqueries with LOOSESCAN hints create invalid query plan

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LooseScan strategy doesn't support join orders where tables from two
subqueries interleave. There is a check in LooseScan_picker::check_qep()
to prevent use of LooseScan for such join orders.

However for two subqueries with one table each a join order of
  subquery1_table
  subquery2_table
  outer_table
was not rejected (although the POSITION object for the last table in the
join order clearly had pos->dups_producing_tables!=0).

Modified the no-interleaving check to catch more cases.
This commit is contained in:
Sergei Petrunia 2025-04-07 21:06:06 +03:00 committed by Oleg Smirnov
parent 453a86f68e
commit b89a1e7f35
4 changed files with 61 additions and 5 deletions
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2
mysql-test/main/opt_hints_join_order.result
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@ -1318,7 +1318,7 @@ ta3.f2 IN (SELECT /*+ QB_NAME(qb2) */ f2 FROM t2);
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY t2 index f2 f2 5 NULL 3 100.00 Using where; Using index; Start temporary
1 PRIMARY t4 ALL NULL NULL NULL NULL 1 100.00 Using where; Using join buffer (flat, BNL join)
1 PRIMARY ta3 ref f2 f2 5 test.t2.f2 1 33.33 Using index
1 PRIMARY ta3 ref f2 f2 5 test.t2.f2 1 100.00 Using index
1 PRIMARY ta4 eq_ref PRIMARY PRIMARY 4 test.t4.f1 1 33.33 End temporary
1 PRIMARY t1 ALL NULL NULL NULL NULL 1 100.00 Using join buffer (flat, BNL join)
Warnings:

22
mysql-test/main/opt_hints_subquery_innodb.result Normal file
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@ -0,0 +1,22 @@
#
# MDEV-36169: LooseScan optimization picks invalid plan
#
CREATE TABLE t1 (a INTEGER NOT NULL, b INT, UNIQUE KEY (a)) engine=InnoDB;
CREATE TABLE t2 (a INTEGER NOT NULL, KEY (a));
CREATE TABLE t3 (a INTEGER NOT NULL, b INT, KEY (a));
INSERT INTO t1 VALUES (1,10), (2,20), (3,30), (4,40);
INSERT INTO t2 VALUES (2), (2), (3), (3), (4), (5);
INSERT INTO t3 VALUES (10,3), (15,3), (20,4), (30,5);
# Must not be: t3, t3(LooseScan), t1
EXPLAIN EXTENDED
SELECT * FROM t1
WHERE
t1.a IN (SELECT /*+ SEMIJOIN(LOOSESCAN) */ a FROM t2) AND
t1.b IN (SELECT /*+ SEMIJOIN(LOOSESCAN) */ a FROM t3);
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY t2 index a a 4 NULL 6 100.00 Using index; LooseScan
1 PRIMARY t1 eq_ref a a 4 test.t2.a 1 16.67 Using where
1 PRIMARY t3 ref a a 4 test.t1.b 1 100.00 Using index; Start temporary; End temporary
Warnings:
Note 1003 select /*+ SEMIJOIN(@`select#2` LOOSESCAN) SEMIJOIN(@`select#3` LOOSESCAN) */ `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1` semi join (`test`.`t2`) semi join (`test`.`t3`) where `test`.`t1`.`a` = `test`.`t2`.`a` and `test`.`t3`.`a` = `test`.`t1`.`b`
DROP TABLE t1,t2,t3;

21
mysql-test/main/opt_hints_subquery_innodb.test Normal file
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@ -0,0 +1,21 @@
--echo #
--echo # MDEV-36169: LooseScan optimization picks invalid plan
--echo #
--source include/have_innodb.inc
CREATE TABLE t1 (a INTEGER NOT NULL, b INT, UNIQUE KEY (a)) engine=InnoDB;
CREATE TABLE t2 (a INTEGER NOT NULL, KEY (a));
CREATE TABLE t3 (a INTEGER NOT NULL, b INT, KEY (a));
INSERT INTO t1 VALUES (1,10), (2,20), (3,30), (4,40);
INSERT INTO t2 VALUES (2), (2), (3), (3), (4), (5);
INSERT INTO t3 VALUES (10,3), (15,3), (20,4), (30,5);
--echo # Must not be: t3, t3(LooseScan), t1
EXPLAIN EXTENDED
SELECT * FROM t1
WHERE
t1.a IN (SELECT /*+ SEMIJOIN(LOOSESCAN) */ a FROM t2) AND
t1.b IN (SELECT /*+ SEMIJOIN(LOOSESCAN) */ a FROM t3);
DROP TABLE t1,t2,t3;

21
sql/opt_subselect.cc
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@ -3402,11 +3402,24 @@ bool LooseScan_picker::check_qep(JOIN *join,
then
stop considering loose scan
*/
if ((first_loosescan_table != MAX_TABLES) && // (1)
(first->table->emb_sj_nest->sj_inner_tables & remaining_tables) && //(2)
new_join_tab->emb_sj_nest != first->table->emb_sj_nest) //(2)
if (first_loosescan_table != MAX_TABLES)
//(first->table->emb_sj_nest->sj_inner_tables & remaining_tables) && //(2)
{
first_loosescan_table= MAX_TABLES;
bool interleaving=false;
if (new_join_tab->emb_sj_nest)
{
interleaving=
MY_TEST(new_join_tab->emb_sj_nest != first->table->emb_sj_nest);
}
else
{
interleaving= (first->table->emb_sj_nest->sj_inner_tables & remaining_tables);
}
if (interleaving)
{
first_loosescan_table= MAX_TABLES;
return FALSE;
}
}
/*