mysql ignore index for order by on join - mysql

Question

CREATE TABLE `call_session` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`contact_id` bigint(20) unsigned NOT NULL,
`campaign_id` bigint(20) unsigned DEFAULT NULL,
`user_id` bigint(20) unsigned DEFAULT NULL,
`start_time` datetime DEFAULT NULL,
`end_time` datetime DEFAULT NULL,
`type` varchar(16) DEFAULT NULL,
`status` varchar(16) DEFAULT NULL,
`continue_reason_id` bigint(20) unsigned DEFAULT NULL,
`continue_by` bigint(20) unsigned DEFAULT NULL,
`end_reason_id` bigint(20) unsigned DEFAULT NULL,
`comment` varchar(255) DEFAULT '',
`call_count` int(11) NOT NULL DEFAULT '0',
`answered_call_count` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
KEY `ix_call_session_contact_id` (`contact_id`),
KEY `ix_call_session_user_id` (`user_id`),
KEY `ix_call_session_campaign_id` (`campaign_id`),
KEY `call_session_continue_by_foreign` (`continue_by`),
KEY `call_session_end_reason_id_foreign` (`end_reason_id`),
KEY `ix_call_session_end_time` (`end_time`),
KEY `ix_call_session_start_time` (`start_time`),
KEY `ix_call_session_continue_reason_id` (`continue_reason_id`),
CONSTRAINT `call_session_campaign_id_foreign` FOREIGN KEY (`campaign_id`) REFERENCES `campaign` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `call_session_continue_by_foreign` FOREIGN KEY (`continue_by`) REFERENCES `user` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `call_session_continue_reason_id_foreign` FOREIGN KEY (`continue_reason_id`) REFERENCES `continue_reason` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `call_session_end_reason_id_foreign` FOREIGN KEY (`end_reason_id`) REFERENCES `end_reason` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `call_session_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE SET NULL ON UPDATE CASCADE
)
CREATE TABLE `continue_reason` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
`project_id` bigint(20) unsigned DEFAULT NULL,
`sort` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
KEY `ix_continue_reason_project_id` (`project_id`),
CONSTRAINT `continue_reason_project_id_foreign` FOREIGN KEY (`project_id`) REFERENCES `project` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
)
The call_session table contain more than 500,000 rows.
The continue_reason table contain only one row.
When I join the continue_reason table for the name column, mysql ignore the start_date index so the query takes between 1.8 to 3 seconds.
I can't figure out why.
The problematic query is:
SELECT `call_session`.`id`,
`continue_reason`.`name`
FROM `call_session`
LEFT JOIN `continue_reason` ON `continue_reason`.`id` = `call_session`.`continue_reason_id`
ORDER BY `call_session`.`start_time` DESC LIMIT 1;
Explain result:
Same query without join works great:
SELECT `call_session`.`id`,
(SELECT `name` FROM `continue_reason` WHERE id = `call_session`.`continue_reason_id`) AS `continue_reason.name`
FROM `call_session`
ORDER BY `call_session`.`start_time` DESC LIMIT 1;
Explain results:
I found many similar questions but I don't have any DISTINCT, no GROUP BY, and there is a very clear benefit for using the index.
Any thoughts?
Thanks in advance
P.S. Force index didn't help.
Here is the Optimizer report