Why isn't the index being used?

[Jim Daemon]

I have the following table:

CREATE TABLE `ftboscaff_rt_session` (
`session_id` varchar(50) NOT NULL default '',
`affiliate_id` bigint(20) unsigned default NULL,
`affiliate_commission` decimal(3,2) default NULL,
`gui_id` bigint(20) default NULL,
`tracking_id` bigint(20) default NULL,
`slave_id` bigint(20) unsigned default NULL,
`tracking_commission` decimal(3,2) default NULL,
`slave_commission` decimal(3,2) default NULL,
`modified` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
`master_id` bigint(20) default NULL,
`master_commission` decimal(3,2) default NULL,
PRIMARY KEY (`session_id`),
KEY `gui_id` (`gui_id`),
KEY `tracking_id` (`tracking_id`),
KEY `affiliate_id` (`affiliate_id`),
KEY `master_id` (`master_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

and executing the following query:
SELECT session_id, affiliate_id, affiliate_commission, gui_id, tracking_id, slave_id, tracking_commission, slave_commission, modified, master_id, master_commission, UNIX_TIMESTAMP( modified ) AS modified_t, UNIX_TIMESTAMP( ) AS now_t
FROM `ftboscaff_rt_session`
WHERE `session_id` = 'ffdde1b419d42c3ba6c7b856bd738a1b'
LIMIT 0 , 30

and according to EXPLAIN the primary index is not being used:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE ftboscaff_rt_session system PRIMARY NULL NULL NULL 1

Any ideas why the index is not being used and how I can correct that.

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