Greatly increased query time after update to 4.2.9

I've just updated the development instance of our RT service, which had been running at 4.0.8, to the latest 4.2.9 in preparation for a planned production update to follow. Unfortunately, we've hit a significant snag: the UI is now hugely slow in some views that were relatively snappy before the update. The slow query log identifies a likely culprit:


# Query_time: 23  Lock_time: 0  Rows_sent: 28  Rows_examined: 858133
SELECT DISTINCT main.* FROM Users main CROSS JOIN ACL ACL_3 JOIN Principals Principals_1  ON ( Principals_1.id = main.id ) JOIN CachedGroupMembers CachedGroupMembers_2  ON ( CachedGroupMembers_2.MemberId = Principals_1.id ) JOIN CachedGroupMembers CachedGroupMembers_4  ON ( CachedGroupMembers_4.MemberId = Principals_1.id )  WHERE ((ACL_3.ObjectType = 'RT::Queue' AND ACL_3.ObjectId   = 3) OR (ACL_3.ObjectType = 'RT::System' AND ACL_3.ObjectId   = 1)) AND (ACL_3.PrincipalId = CachedGroupMembers_4.GroupId) AND (ACL_3.PrincipalType = 'Group') AND (ACL_3.RightName = 'OwnTicket' OR ACL_3.RightName = 'SuperUser') AND (CachedGroupMembers_2.Disabled = '0') AND (CachedGroupMembers_2.GroupId = '4') AND (CachedGroupMembers_4.Disabled = '0') AND (Principals_1.Disabled = '0') AND (Principals_1.PrincipalType = 'User') AND (Principals_1.id != '1')  ORDER BY main.Name ASC;


It appears that this query is reasonably well-indexed, but clearly something has changed in the schema updates between 4.0.8 and 4.2.9:


mysql> explain extended SELECT DISTINCT main.* FROM Users main CROSS JOIN ACL ACL_3 JOIN Principals Principals_1  ON ( Principals_1.id = main.id ) JOIN CachedGroupMembers CachedGroupMembers_2  ON ( CachedGroupMembers_2.MemberId = Principals_1.id ) JOIN CachedGroupMembers CachedGroupMembers_4  ON ( CachedGroupMembers_4.MemberId = Principals_1.id )  WHERE ((ACL_3.ObjectType = 'RT::Queue' AND ACL_3.ObjectId   = 3) OR (ACL_3.ObjectType = 'RT::System' AND ACL_3.ObjectId   = 1)) AND (ACL_3.PrincipalId = CachedGroupMembers_4.GroupId) AND (ACL_3.PrincipalType = 'Group') AND (ACL_3.RightName = 'OwnTicket' OR ACL_3.RightName = 'SuperUser') AND (CachedGroupMembers_2.Disabled = '0') AND (CachedGroupMembers_2.GroupId = '4') AND (CachedGroupMembers_4.Disabled = '0') AND (Principals_1.Disabled = '0') AND (Principals_1.PrincipalType = 'User') AND (Principals_1.id != '1')  ORDER BY main.Name ASC;
+----+-------------+----------------------+--------+----------------------------------------------------+---------------------+---------+------------------------------------+------+-----------------------------------------------------------+
| id | select_type | table                | type   | possible_keys                                      | key                 | key_len | ref                                | rows | Extra                                                     |
+----+-------------+----------------------+--------+----------------------------------------------------+---------------------+---------+------------------------------------+------+-----------------------------------------------------------+
|  1 | SIMPLE      | CachedGroupMembers_2 | range  | DisGrouMem,CachedGroupMembers3,cachedgroupmembers1 | DisGrouMem          | 10      | NULL                               |  606 | Using where; Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | main                 | eq_ref | PRIMARY                                            | PRIMARY             | 4       | rt34.CachedGroupMembers_2.MemberId |    1 |                                                           |
|  1 | SIMPLE      | Principals_1         | eq_ref | PRIMARY                                            | PRIMARY             | 4       | rt34.main.id                       |    1 | Using where; Distinct                                     |
|  1 | SIMPLE      | CachedGroupMembers_4 | ref    | DisGrouMem,CachedGroupMembers3,cachedgroupmembers1 | CachedGroupMembers3 | 5       | rt34.CachedGroupMembers_2.MemberId |    1 | Using where; Distinct                                     |
|  1 | SIMPLE      | ACL_3                | range  | ACL1                                               | ACL1                | 85      | NULL                               |   13 | Using where; Using index; Distinct                        |
+----+-------------+----------------------+--------+----------------------------------------------------+---------------------+---------+------------------------------------+------+-----------------------------------------------------------+
5 rows in set, 1 warning (0.00 sec)


Can anyone offer any advice on what might have precipitated this sudden drop in query performance?


Cheers,

Jeff