The following test case extracted from subselect.test, results in the
allocation of a 4 GB memory buffer for JOIN_CACHE. This huge buffer
is allocated because of subtracting bigger record length from a smaller
one, then assigning to a UINT. The resulting negative value overflows
and results in a very big positive number.

Test case:

CREATE TABLE t1 (
categoryId int(11) NOT NULL,
courseId int(11) NOT NULL,
startDate datetime NOT NULL,
endDate datetime NOT NULL,
createDate datetime NOT NULL,
modifyDate timestamp NOT NULL,
attributes text NOT NULL
);
INSERT INTO t1 VALUES (1,41,'2004-02-09','2010-01-01','2004-02-09','2004-02-09',''),
(1,86,'2004-08-16','2004-08-16','2004-08-16','2004-08-16',''),
(1,87,'2004-08-16','2004-08-16','2004-08-16','2004-08-16',''),
(2,52,'2004-03-15','2004-10-01','2004-03-15','2004-09-17',''),
(2,53,'2004-03-16','2004-10-01','2004-03-16','2004-09-17',''),
(2,88,'2004-08-16','2004-08-16','2004-08-16','2004-08-16',''),
(2,89,'2004-08-16','2004-08-16','2004-08-16','2004-08-16',''),
(3,51,'2004-02-09','2010-01-01','2004-02-09','2004-02-09',''),
(5,12,'2004-02-18','2010-01-01','2004-02-18','2004-02-18','');

CREATE TABLE t2 (
userId int(11) NOT NULL,
courseId int(11) NOT NULL,
date datetime NOT NULL
);
INSERT INTO t2 VALUES (5141,71,'2003-11-18'),
(5141,72,'2003-11-25'),(5141,41,'2004-08-06'),
(5141,52,'2004-08-06'),(5141,53,'2004-08-06'),
(5141,12,'2004-08-06'),(5141,86,'2004-10-21'),
(5141,87,'2004-10-21'),(5141,88,'2004-10-21'),
(5141,89,'2004-10-22'),(5141,51,'2004-10-26');

CREATE TABLE t3 (
groupId int(11) NOT NULL,
parentId int(11) NOT NULL,
startDate datetime NOT NULL,
endDate datetime NOT NULL,
createDate datetime NOT NULL,
modifyDate timestamp NOT NULL,
ordering int(11)
);
INSERT INTO t3 VALUES (12,9,'1000-01-01','3999-12-31','2004-01-29','2004-01-29',NULL);

CREATE TABLE t4 (
id int(11) NOT NULL,
groupTypeId int(11) NOT NULL,
groupKey varchar(50) NOT NULL,
name text,
ordering int(11),
description text,
createDate datetime NOT NULL,
modifyDate timestamp NOT NULL
);
INSERT INTO t4 VALUES (9,5,'stationer','stationer',0,'Stationer','2004-01-29','2004-01-29'),
(12,5,'group2','group2',0,'group2','2004-01-29','2004-01-29');

CREATE TABLE t5 (
userId int(11) NOT NULL,
groupId int(11) NOT NULL,
createDate datetime NOT NULL,
modifyDate timestamp NOT NULL
);
INSERT INTO t5 VALUES (5141,12,'2004-08-06','2004-08-06');

select
count(distinct t2.userid) pass,
groupstuff.*,
count(t2.courseid) crse,
t1.categoryid,
t2.courseid,
date_format(date, '%b%y') as colhead
from t2
join t1 on t2.courseid=t1.courseid
join
(
select
t5.userid,
parentid,
parentgroup,
childid,
groupname,
grouptypeid
from t5
join
(
select t4.id as parentid,
t4.name as parentgroup,
t4.id as childid,
t4.name as groupname,
t4.grouptypeid
from t4
) as gin on t5.groupid=gin.childid
) as groupstuff on t2.userid = groupstuff.userid
group by
groupstuff.groupname, colhead , t2.courseid;