I'm trying to join 2 fairly large tables. One of which I've put a compound index on one of them, to try and speed things up. However I can't seem to get the query to utilise the compound index.

Could someone suggest what changes I could make to the indexes or the query to make it run any faster (currently about 0.75s. I'd like it faster).
CREATE TABLE `product_group` (
  `prodGroupID` bigint(20) unsigned NOT NULL auto_increment,
  `productID` bigint(20) unsigned default NULL,
  `groupType` varchar(10) default NULL,
  `groupID` bigint(20) unsigned default NULL,
  PRIMARY KEY  (`prodGroupID`),
  KEY `productID` (`productID`),
  KEY `groupID` (`groupID`),
  KEY `groupType` (`groupType`),
  KEY `type_id` (`groupType`,`groupID`)

CREATE TABLE `sale_date_by_group` (
  `unique_key` varchar(100) NOT NULL default '',
  `groupType` varchar(10) default NULL,
  `groupID` int(11) default NULL,
  `date` date default NULL,
  `salesChannelID` double(5,1) default NULL,
  `units` int(11) default NULL,
  `turnover` double(10,2) default NULL,
  `cost` double(10,2) default NULL,
  PRIMARY KEY  (`unique_key`),
  KEY `date` (`date`),
  KEY `salesChannelID` (`salesChannelID`),
  KEY `combined_index` (`groupID`,`groupType`,`date`,`salesChannelID`)

SELECT pg.productID, SUM(s.units)units FROM 
sale_date_by_group s, product_group pg 
pg.groupType='store' AND 
pg.groupID=42 AND 
pg.productID=s.groupID AND 
s.groupType='product' AND 
GROUP BY pg.productID
order by units desc limit 5

The EXPLAIN outputs:
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	s	range	date,combined_index	date	4	NULL	463359	Using where; Using temporary; Using filesort
1	SIMPLE	pg	ref	productID,groupID,groupType,type_id	productID	9	hsdmain.s.groupID	1	Using where
you can see from the EXPLAIN results that the combined index on sale_date_by_group isn't being utilised.

Any pointers on improving the indexing or the query itself to make it faster?