index is not used why?

[Thomas Jüntgen]

Hello all

I have this table with more then 15000 rows.
CREATE TABLE `t_event_calendar` (
`c_id` varchar(45) NOT NULL default '',
`d_start` datetime NOT NULL default '0000-00-00 00:00:00',
`d_start_aktuell` datetime default '0000-00-00 00:00:00',
`d_ende_aktuell` datetime default '0000-00-00 00:00:00',
`d_ende` datetime NOT NULL default '0000-00-00 00:00:00',
`n_typ_id` int(11) NOT NULL default '0',
`n_optionen` int(11) NOT NULL default '0',
`n_status` int(11) default NULL,
`n_label_farbe` int(11) default NULL,
`c_ort` varchar(45) default NULL,
`n_kunde_id` int(11) default NULL,
`c_notiz` text,
`c_parent_id` varchar(45) default NULL,
`n_recurrence_idx` int(11) default NULL,
`c_text` varchar(45) NOT NULL default '',
`n_resource_id` varchar(45) default NULL,
`n_group_id` int(11) default NULL,
`n_internal_event_type` int(11) NOT NULL default '0',
`n_reminder_min` int(11) default NULL COMMENT 'the minutes before start time when the reminder should appear',
`d_reminder_date` datetime default NULL,
`n_recurrence_info` text,
`c_task_id` varchar(40) default NULL COMMENT 't_event_task',
`m_keys` text COMMENT 'db keys for faster access',
`n_public` tinyint(1) NOT NULL default '1',
`n_erledigt` tinyint(1) NOT NULL default '0',
PRIMARY KEY (`c_id`),
KEY `IDX_START_RESOURCE` (`d_start`,`n_resource_id`),
KEY `IDX_RESOURCE_GROUP` (`n_resource_id`,`n_group_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 ROW_FORMAT=FIXED;

This select is not using the index. Instead it using the type 'ALL'.
SELECT *
FROM t_event_calendar
WHERE (n_resource_id = 4 OR n_group_id IN (1) OR (n_group_id IS NULL AND n_resource_id IS NULL))

Could any onel help me?
Thanks Thomas