Bug #12072 Incorrect results when using GROUP BY after applying an index
Submitted: 20 Jul 2005 21:02 Modified: 21 Jul 2005 0:38
Reporter: . . Email Updates:
Status: Closed Impact on me:
None 
Category:MySQL Server Severity:S1 (Critical)
Version:5.0.9 OS:Microsoft Windows (WinXP)
Assigned to: CPU Architecture:Any

[20 Jul 2005 21:02] . .
Description:
SELECT statement incorrectly returns an empty result set when using GROUP BY after applying an index to the table.

How to repeat:
Create a test schema and database:

CREATE DATABASE zootest;
USE zootest;
CREATE TABLE `zoo` (
  `animal_id` int(11) NOT NULL,
  `species` varchar(128) NOT NULL,
  PRIMARY KEY  (`animal_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO `zoo` (`animal_id`,`species`) VALUES 
 (1,'monkey'),
 (2,'monkey'),
 (3,'monkey'),
 (4,'monkey'),
 (5,'monkey'),
 (6,'elephant'),
 (7,'elephant'),
 (8,'elephant'),
 (9,'lion'),
 (10,'giraffe');

Now you can run this SELECT statement:

SELECT species, COUNT(animal_id) AS countanimal_id FROM zoo
WHERE species NOT LIKE 'monkey%'
GROUP BY species ORDER BY species;

and you will get the expected results:
+----------+----------------+
| species  | countanimal_id |
+----------+----------------+
| elephant |              3 |
| giraffe  |              1 |
| lion     |              1 |
+----------+----------------+
3 rows in set (0.00 sec)

Now apply an index to the table:

ALTER TABLE `zootest`.`zoo` ADD INDEX `Index_2`(`species`);

and run the same SELECT statement again:

SELECT species, COUNT(animal_id) AS countanimal_id FROM zoo
WHERE species NOT LIKE 'monkey%'
GROUP BY species ORDER BY species;

and you will incorrectly get an empty result set:

Empty set (0.00 sec)

I can reproduce the error using the command line interface and MySQL Administrator.
[20 Jul 2005 21:12] . .
MySQL Administrator Backup dump of the database before applying the index

Attachment: zootest.sql (text/plain), 1.69 KiB.

[21 Jul 2005 0:38] MySQL Verification Team
Thank you for your bug report. This issue has been committed to our
source repository of that product and will be incorporated into the
next release.

If necessary, you can access the source repository and build the latest
available version, including the bugfix, yourself. More information 
about accessing the source trees is available at
    http://www.mysql.com/doc/en/Installing_source_tree.html

Additional info:

c:\mysql\bin>mysql -uroot test
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1 to server version: 5.0.10-beta-nt

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> CREATE DATABASE zootest;
Query OK, 1 row affected (0.03 sec)

mysql> USE zootest;
Database changed
mysql> CREATE TABLE `zoo` (
    ->   `animal_id` int(11) NOT NULL,
    ->   `species` varchar(128) NOT NULL,
    ->   PRIMARY KEY  (`animal_id`)
    -> ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Query OK, 0 rows affected (0.11 sec)

mysql> INSERT INTO `zoo` (`animal_id`,`species`) VALUES
    ->  (1,'monkey'),
    ->  (2,'monkey'),
    ->  (3,'monkey'),
    ->  (4,'monkey'),
    ->  (5,'monkey'),
    ->  (6,'elephant'),
    ->  (7,'elephant'),
    ->  (8,'elephant'),
    ->  (9,'lion'),
    ->  (10,'giraffe');
Query OK, 10 rows affected (0.02 sec)
Records: 10  Duplicates: 0  Warnings: 0

mysql> SELECT species, COUNT(animal_id) AS countanimal_id FROM zoo
    -> WHERE species NOT LIKE 'monkey%'
    -> GROUP BY species ORDER BY species;
+----------+----------------+
| species  | countanimal_id |
+----------+----------------+
| elephant |              3 |
| giraffe  |              1 |
| lion     |              1 |
+----------+----------------+
3 rows in set (0.02 sec)

mysql> ALTER TABLE `zootest`.`zoo` ADD INDEX `Index_2`(`species`);
Query OK, 10 rows affected (0.19 sec)
Records: 10  Duplicates: 0  Warnings: 0

mysql> SELECT species, COUNT(animal_id) AS countanimal_id FROM zoo
    -> WHERE species NOT LIKE 'monkey%'
    -> GROUP BY species ORDER BY species;
+----------+----------------+
| species  | countanimal_id |
+----------+----------------+
| elephant |              3 |
| giraffe  |              1 |
| lion     |              1 |
+----------+----------------+
3 rows in set (0.02 sec)