MySQL use separate indices for JOIN and GROUP BY - mysql

Question

I am trying to execute following query
SELECT
a.sessionID AS `sessionID`,
firstSeen, birthday, gender,
isAnonymous, LanguageCode
FROM transactions AS trx
INNER JOIN actions AS a ON a.sessionID = trx.SessionID
WHERE a.ActionType = 'PURCHASE'
GROUP BY trx.TransactionNumber
Explain provides the following output
1 SIMPLE trx ALL TransactionNumber,SessionID NULL NULL NULL 225036 Using temporary; Using filesort
1 SIMPLE a ref sessionID sessionID 98 infinitiExport.trx.SessionID 1 Using index
The problem is that I am trying to use one field for join and different field for GROUP BY.
How can I tell MySQL to use different indices for same table?
CREATE TABLE `transactions` (
`SessionID` varchar(32) NOT NULL DEFAULT '',
`date` datetime DEFAULT NULL,
`TransactionNumber` varchar(32) NOT NULL DEFAULT '',
`CustomerECommerceTrackID` int(11) DEFAULT NULL,
`SKU` varchar(45) DEFAULT NULL,
`AmountPaid` double DEFAULT NULL,
`Currency` varchar(10) DEFAULT NULL,
`Quantity` int(11) DEFAULT NULL,
`Name` tinytext NOT NULL,
`Category` varchar(45) NOT NULL DEFAULT '',
`customerInfoXML` text,
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`),
KEY `TransactionNumber` (`TransactionNumber`),
KEY `SessionID` (`SessionID`)
) ENGINE=InnoDB AUTO_INCREMENT=212007 DEFAULT CHARSET=utf8;
CREATE TABLE `actions` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`sessionActionDate` datetime DEFAULT NULL,
`actionURL` varchar(255) DEFAULT NULL,
`sessionID` varchar(32) NOT NULL DEFAULT '',
`ActionType` varchar(64) DEFAULT NULL,
`CustomerID` int(11) DEFAULT NULL,
`IPAddressID` int(11) DEFAULT NULL,
`CustomerDeviceID` int(11) DEFAULT NULL,
`customerInfoXML` text,
PRIMARY KEY (`id`),
KEY `ActionType` (`ActionType`),
KEY `CustomerDeviceID` (`CustomerDeviceID`),
KEY `sessionID` (`sessionID`)
) ENGINE=InnoDB AUTO_INCREMENT=15042833 DEFAULT CHARSET=utf8;
Thanks
EDIT 1: My indexes were broken, I had to add (SessionID, TransactionNumber) index to transactions table, however now, when I try to include trx.customerInfoXML table mysql stops using index
EDIT 2 Another answer does not really solved my problem because it's not standard sql syntax and generally not a good idea to force indices.
For ORM users such syntax is a unattainable luxury.
EDIT 3 I updated my indices and it solved the problem, see EDIT 1