Wrong result (missing rows) with materialization+semijoin+join_cache_hashed, join_cache_level>2, HAVING

[Elena Stepanova]

The following query

SELECT a FROM t1
WHERE a IN (
  SELECT b FROM t1, t2
  WHERE b = a
)
GROUP BY a
HAVING a != 'z'

on the test data returns only one row if it's run with materialization, semijoin, join_cache_hashed and join_cache_level>2, and two rows otherwise. The latter is correct.

bzr version-info
revision-id: <email address hidden>
date: 2012-02-16 20:15:57 +0400
build-date: 2012-02-17 21:19:42 +0400
revno: 3424

EXPLAIN with join_cache_level = 3 (wrong result):

id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY <subquery2> ALL distinct_key NULL NULL NULL 2 100.00 Using temporary; Using filesort
1 PRIMARY t1 hash_index a #hash#a:a 4:4 test.t1.a 3 66.67 Using where; Using join buffer (flat, BNLH join)
2 MATERIALIZED t2 ALL NULL NULL NULL NULL 2 100.00 Using where
2 MATERIALIZED t1 hash_index a #hash#a:a 4:4 test.t2.b 3 66.67 Using where; Using join buffer (flat, BNLH join)
Warnings:
Note 1003 select `test`.`t1`.`a` AS `a` from `test`.`t1` semi join (`test`.`t1` join `test`.`t2`) where ((`test`.`t1`.`a` = `test`.`t2`.`b`) and (`test`.`t1`.`a` = `test`.`t2`.`b`)) group by `test`.`t1`.`a` having (`test`.`t1`.`a` <> 'z')

EXPLAIN with join_cache_level =2 (correct result):

id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY <subquery2> ALL distinct_key NULL NULL NULL 2 100.00 Using temporary; Using filesort
1 PRIMARY t1 ref a a 4 test.t2.b 2 100.00 Using index
2 MATERIALIZED t2 ALL NULL NULL NULL NULL 2 100.00 Using where
2 MATERIALIZED t1 ref a a 4 test.t2.b 2 100.00 Using index
Warnings:
Note 1003 select `test`.`t1`.`a` AS `a` from `test`.`t1` semi join (`test`.`t1` join `test`.`t2`) where ((`test`.`t1`.`a` = `test`.`t2`.`b`) and (`test`.`t1`.`a` = `test`.`t2`.`b`)) group by `test`.`t1`.`a` having (`test`.`t1`.`a` <> 'z')

Minimal optimizer_switch: materialization=on,semijoin=on,join_cache_hashed=on
(and join_cache_level>=3)
Full optimizer_switch: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=on,firstmatch=on,loosescan=on,materialization=on,in_to_exists=on,semijoin=on,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=on,semijoin_with_cache=on,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on

# Test case:

SET optimizer_switch = 'materialization=on,semijoin=on,join_cache_hashed=on';
# join_cache_level >= 3
SET join_cache_level = 3;

# MyISAM or Aria
CREATE TABLE t1 ( a VARCHAR(1), KEY(a) ) ENGINE=MyISAM;
INSERT INTO t1 VALUES ('c'),('v'),('c');
CREATE TABLE t2 ( b VARCHAR(1) ) ENGINE=MyISAM;
INSERT INTO t2 VALUES ('v'),('c');

SELECT a FROM t1
WHERE a IN (
  SELECT b FROM t1, t2
  WHERE b = a
)
GROUP BY a
HAVING a != 'z';

# End of test case

# Expected result:
# a
# ---
# c
# v

# Actual result:
# a
# ---
# c

# There were 6 other similar cases during the test run.

[Igor Babaev (igorb-seattle)]

The problem is seen already in the execution plan:

MariaDB [test]> EXPLAIN SELECT a FROM t1 t WHERE t.a IN ( SELECT b FROM t1, t2 WHERE b = a ) GROUP BY t.a HAVING t.a != 'z';
+----+--------------+-------------+------+---------------+------+---------+-----------+------+----------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+------+---------------+------+---------+-----------+------+----------+---------------------------------+
| 1 | PRIMARY | <subquery2> | ALL | distinct_key | NULL | NULL | NULL | 2 | 100.00 | Using temporary; Using filesort |
| 1 | PRIMARY | t | ref | a | a | 4 | test.t1.a | 2 | 100.00 | Using where; Using index |
| 2 | MATERIALIZED | t2 | ALL | NULL | NULL | NULL | NULL | 2 | 100.00 | Using where |
| 2 | MATERIALIZED | t1 | ref | a | a | 4 | test.t2.b | 2 | 100.00 | Using where; Using index |
+----+--------------+-------------+------+---------------+------+---------+-----------+------+----------+-------------------

The key to access t is built oveer column a of the materialized table. Yet there is no such column there.

[Igor Babaev (igorb-seattle)]

The previous plan was obtained with the following settings:

SET optimizer_switch = 'materialization=on,semijoin=on,join_cache_hashed=on';
SET join_cache_level = 6;

[Elena Stepanova (elenst)]

Fix released with 5.3.5-ga.