Uploaded image for project: 'MariaDB Server'
  1. MariaDB Server
  2. MDEV-8256

A part of a ROW comparison is erroneously optimized away

        Details

        • Type: Bug
        • Status: Closed
        • Priority: Major
        • Resolution: Fixed
        • Affects Version/s: 10.1, 10.0
        • Fix Version/s: 10.1.6
        • Component/s: Optimizer
        • Labels:
          None
        • Sprint:
          10.1.6-1

          Description

          This script:

          DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET utf8);
INSERT INTO t1 VALUES ('1e1'),('1é1');
SELECT * FROM t1 WHERE a=10;
SELECT * FROM t1 WHERE a='1e1';
SELECT * FROM t1 WHERE a=10 AND a='1e1';

          returns:

          MariaDB [test]> SELECT * FROM t1 WHERE a=10;
+------+
| a    |
+------+
| 1e1  |
+------+
1 row in set, 1 warning (0.00 sec)

MariaDB [test]> SELECT * FROM t1 WHERE a='1e1';
+------+
| a    |
+------+
| 1e1  |
| 1é1  |
+------+
2 rows in set (0.00 sec)

MariaDB [test]> SELECT * FROM t1 WHERE a=10 AND a='1e1';
+------+
| a    |
+------+
| 1e1  |
+------+
1 row in set, 1 warning (0.00 sec)

          Looks good so far.

          Now if I rewrite the last query using ROW syntax:

          SELECT * FROM t1 WHERE (a,a)=(10,'1e1');

          It erroneously starts to return both rows:

          +------+
| a    |
+------+
| 1e1  |
| 1é1  |
+------+

          This EXPLAIN EXTENDED:

          MariaDB [test]> EXPLAIN SELECT * FROM t1 WHERE (a,a)=(10,'1e1');
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

MariaDB [test]> SHOW WARNINGS;
Empty set (0.00 sec)

MariaDB [test]> EXPLAIN EXTENDED SELECT * FROM t1 WHERE (a,a)=(10,'1e1');
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

MariaDB [test]> SHOW WARNINGS;
+-------+------+--------------------------------------------------------------------------------+
| Level | Code | Message                                                                        |
+-------+------+--------------------------------------------------------------------------------+
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = '1e1') |
+-------+------+--------------------------------------------------------------------------------+
1 row in set (0.00 sec)

          tells that the a=10 part was moved away from the condition.
          This is wrong. It should be preserved.

            Gliffy Diagrams

              Attachments

                Activity

                There are no comments yet on this issue.

                  People

                  • Assignee:
                    bar Alexander Barkov
                    Reporter:
                    bar Alexander Barkov
                  • Votes:
                    0 Vote for this issue
                    Watchers:
                    2 Start watching this issue

                    Dates

                    • Created:
                      Updated:
                      Resolved:

                      Agile