Query with GROUP BY returns an extra row

[Igor Babaev]

The following test case produces a wrong result in MariaDB 5.2:

CREATE TABLE t1 (vc varchar(1), INDEX vc_idx (vc)) ;
INSERT INTO t1 VALUES (NULL), ('o'), (NULL), ('p'), ('c');
FLUSH TABLE t1;
SELECT vc FROM t1 GROUP BY vc;

MariaDB [test]> SELECT vc FROM t1 GROUP BY vc;
+------+
| vc |
+------+
| NULL |
| NULL |
| c |
| o |
| p |
+------+
5 rows in set (0.01 sec)

(See also bug #12837714 in mysql-trunk)

[Oleksandr "Sanja" Byelkin (sanja-byelkin)]

It is not repeatable on 5.3

[Oleksandr "Sanja" Byelkin (sanja-byelkin)]

In 5.3 it was fixed by adding this to Cached_item_field::cmp :

 /*
    If value is not null and value changed (from null to not null or
    becasue of value change), then copy the new value to buffer.
    */
  if (! null_value && (tmp || (tmp= (field->cmp(buff) != 0))))
    field->get_image(buff,length,field->charset());