Description:
Bug happens only under following conditions (all of them must be true):
* InnoDb table
* column used in group by or select distinct is indexed with non unique Key
* There is Primary Key on another column
First time when query is invoked there's hign chance to get correct result.
For subsecuent runs it return various "distributions" for NULLs like (4, 1, 1, 1) (5, 1,
1) (4, 2, 1)
How to repeat:
CREATE TABLE t1 (
id int(11) NOT NULL default '0',
n int(11) default NULL,
PRIMARY KEY (id),
KEY n (n)
) TYPE=InnoDB;
INSERT INTO t1 VALUES (1,NULL);
INSERT INTO t1 VALUES (2,NULL);
INSERT INTO t1 VALUES (3,NULL);
INSERT INTO t1 VALUES (5,NULL);
INSERT INTO t1 VALUES (234,NULL);
INSERT INTO t1 VALUES (298,NULL);
INSERT INTO t1 VALUES (4,1);
# Warning. Execute query more than once. Sometimes
# upon first run it returns correct result, but for
# subsequent runs result is wrong
mysql> select n, count(*) from t1 group by n;
+------+----------+
| n | count(*) |
+------+----------+
| NULL | 4 |
| NULL | 2 |
| 1 | 1 |
+------+----------+
3 rows in set (0.02 sec)
mysql> select distinct n from t1;
+------+
| n |
+------+
| NULL |
| NULL |
| 1 |
+------+
3 rows in set (0.06 sec)
