[MDEV-10389] Query returns different results on a debug vs non-debug build of the same revision Created: 2016-07-18  Updated: 2016-07-27  Resolved: 2016-07-27

Status: Closed
Project: MariaDB Server
Component/s: Optimizer
Affects Version/s: 5.5, 10.0, 10.1
Fix Version/s: 10.1.17

Type: Bug Priority: Major
Reporter: Elena Stepanova Assignee: Sergei Petrunia
Resolution: Fixed Votes: 0
Labels: None
Environment:

Jessie x86_64, gcc (Debian 4.9.2-10) 4.9.2, cmake version 3.0.2

Sprint: 10.2.2-3

 Description   

Test case

 
CREATE TABLE t1 (i1 INT, i2 INT NOT NULL) ENGINE=MyISAM;
 
INSERT INTO t1 VALUES (1,4),(2,6);
 
                        
SELECT * FROM t1 AS alias1 
WHERE alias1.i1 IN ( 
    SELECT i1 FROM t1 WHERE alias1.i2 IN ( SELECT i2 FROM t1 HAVING i2 <> 7 )
 
);
 
DROP TABLE t1;

10.0 bf2e31500c debug build

 
MariaDB [test]> SELECT * FROM t1 AS alias1 
    -> WHERE alias1.i1 IN ( 
    ->     SELECT i1 FROM t1 WHERE alias1.i2 IN ( SELECT i2 FROM t1 HAVING i2 <> 7 )
 
    -> );
 
Empty set (0.01 sec)


MariaDB [test]> EXPLAIN EXTENDED
 
    -> SELECT * FROM t1 AS alias1 
    -> WHERE alias1.i1 IN ( 
    ->     SELECT i1 FROM t1 WHERE alias1.i2 IN ( SELECT i2 FROM t1 HAVING i2 <> 7 )
 
    -> );
 
+------+--------------+-------------+--------+---------------+--------------+---------+----------------+------+----------+----------------------------------------------------------------------------+
 
| id   | select_type  | table       | type   | possible_keys | key          | key_len | ref            | rows | filtered | Extra                                                                      |
 
+------+--------------+-------------+--------+---------------+--------------+---------+----------------+------+----------+----------------------------------------------------------------------------+
 
|    1 | PRIMARY      | <subquery3> | eq_ref | distinct_key  | distinct_key | 4       | test.alias1.i2 |    1 |   100.00 |                                                                            |
 
|    1 | PRIMARY      | alias1      | ALL    | NULL          | NULL         | NULL    | NULL           |    2 |   100.00 | Using join buffer (flat, BNL join)                                         |
 
|    1 | PRIMARY      | t1          | ALL    | NULL          | NULL         | NULL    | NULL           |    2 |   100.00 | Using where; FirstMatch(alias1); Using join buffer (incremental, BNL join) |
 
|    3 | MATERIALIZED | t1          | ALL    | NULL          | NULL         | NULL    | NULL           |    2 |   100.00 |                                                                            |
 
+------+--------------+-------------+--------+---------------+--------------+---------+----------------+------+----------+----------------------------------------------------------------------------+
 
4 rows in set, 2 warnings (0.00 sec)
 
 
 
MariaDB [test]> select @@version;
 
+-----------------------+
 
| @@version             |
 
+-----------------------+
 
| 10.0.27-MariaDB-debug |
 
+-----------------------+
 
1 row in set (0.00 sec)

10.0 bf2e31500c relwithdebinfo build

 
MariaDB [test]> SELECT * FROM t1 AS alias1 
    -> WHERE alias1.i1 IN ( 
    ->     SELECT i1 FROM t1 WHERE alias1.i2 IN ( SELECT i2 FROM t1 HAVING i2 <> 7 )
 
    -> );
 
+------+----+
 
| i1   | i2 |
 
+------+----+
 
|    1 |  4 |
 
|    2 |  6 |
 
+------+----+
 
2 rows in set (0.00 sec)


MariaDB [test]> EXPLAIN EXTENDED
 
    -> SELECT * FROM t1 AS alias1 
    -> WHERE alias1.i1 IN ( 
    ->     SELECT i1 FROM t1 WHERE alias1.i2 IN ( SELECT i2 FROM t1 HAVING i2 <> 7 )
 
    -> );
 
+------+--------------+-------------+--------+---------------+--------------+---------+----------------+------+----------+----------------------------------------------------------------------------+
 
| id   | select_type  | table       | type   | possible_keys | key          | key_len | ref            | rows | filtered | Extra                                                                      |
 
+------+--------------+-------------+--------+---------------+--------------+---------+----------------+------+----------+----------------------------------------------------------------------------+
 
|    1 | PRIMARY      | <subquery3> | eq_ref | distinct_key  | distinct_key | 4       | test.alias1.i2 |    1 |   100.00 |                                                                            |
 
|    1 | PRIMARY      | alias1      | ALL    | NULL          | NULL         | NULL    | NULL           |    2 |   100.00 | Using join buffer (flat, BNL join)                                         |
 
|    1 | PRIMARY      | t1          | ALL    | NULL          | NULL         | NULL    | NULL           |    2 |   100.00 | Using where; FirstMatch(alias1); Using join buffer (incremental, BNL join) |
 
|    3 | MATERIALIZED | t1          | ALL    | NULL          | NULL         | NULL    | NULL           |    2 |   100.00 |                                                                            |
 
+------+--------------+-------------+--------+---------------+--------------+---------+----------------+------+----------+----------------------------------------------------------------------------+
 
4 rows in set, 2 warnings (0.00 sec)
 
 
 
MariaDB [test]> select @@version;
 
+-----------------+
 
| @@version       |
 
+-----------------+
 
| 10.0.27-MariaDB |
 
+-----------------+
 
1 row in set (0.00 sec)



 Comments   
Comment by Sergei Petrunia [ 2016-07-26 ]
  • The correct result is the one that returns two records
  • Running debug build with optimizer_switch='materialization=off' makes it produce a correct result.
  • The query plan seems to be incorrect:
    how come the first table 

    |    1 | PRIMARY      | <subquery3> | eq_ref | distinct_key  | distinct_key | 4       | test.alias1.i2

    referes to alias1.i2, if table alias1 is accessed after table <subquery3> ?

Comment by Sergei Petrunia [ 2016-07-27 ]

Debugging...

  • convert_subq_to_jtbm converts subquery#3 into a non-merged semi-join nest
    (aka JTBM)
  • Note that IN-equality of "alias1.i2=t1.i2" doesn't specifically exist for
    JTBMs.
  • convert_subq_to_sj converts subquery#2 to a merged semi-join nest.

The problem is that I don't see where "alias1.i2" had its fix_after_pullout()
call to adjust its attributes.

Indeed, proceeding further to join optimization in the top query, we see:

 
 
(gdb) wher 1
 
  #0  best_access_path (join=0x7fff5400be00, s=0x7fff54030cd8, remaining_tables=6, idx=1, disable_jbuf=false, record_count=2, pos=0x7fff54031360, loose_scan_pos=0x7ffff427c950) at /home/psergey/dev-git/10.1-dbg6/sql/sql_select.cc:5961
 
  (More stack frames follow...)
 
(gdb) p s->table->alias.Ptr
 
  $125 = 0x7fff5402b378 "<subquery3>"
 
(gdb) p dbug_print_item(s->keyuse[0].val)
 
  $126 = 0x555556d53040 "alias1.i2"
 
(gdb) p/x s->keyuse[0].val->used_tables()
 
  $127 = 0x4000000000000000

alias1.i2 has table_map()=OUTER_REF_TABLE_BIT...

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