index_merge full table scan - 2 seconds mysql select - mysql

Question

I have this select:
SELECT MAX(id) FROM chat
WHERE (`to` = 1 and `del_to_status` = '0') or (`from` = 1 and `del_from_status` = '0')
GROUP BY CASE WHEN 1 = `to` THEN `from` ELSE `to` END
chat:
`chat` (
`id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`from` int(11) UNSIGNED NOT NULL,
`to` int(11) UNSIGNED NOT NULL,
`message` text NOT NULL,
`del_from_status` tinyint(1) NOT NULL DEFAULT '0',
`del_to_status` tinyint(1) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
KEY `from` (`from`),
KEY `to` (`to`),
);
The problem is it is using full table scan:
it is taking a lot of time. any ideas to get faster results?

Answer 1

What do you think about this solution:
select grouped_by_to.user, greatest(grouped_by_to.id, grouped_by_from.id ) from
(
select c1.to as user, max(id) as id from chat c1
group by c1.to
) grouped_by_to
join
(
select c1.from as user, max(id) as id from chat c1
group by c1.from
) grouped_by_from on grouped_by_from.user = grouped_by_to.user
Note that i ignored the del_to_status columns, you can add them easily.
But actually I think your whole db schema is wrong, I think you need something more like :
`messages` (
`message_id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`user_id` int(11) UNSIGNED NOT NULL,
`message` text NOT NULL,
`message_date` timestamp NOT NULL,
PRIMARY KEY (`message_id`),
);
`conversatinos` (
`conversation_id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`message_id` int(11) UNSIGNED NOT NULL,
PRIMARY KEY (`conversation_id`),
);
`users` (
`user_id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`user_name` int(11) UNSIGNED NOT NULL,
PRIMARY KEY (`user_id`),
);
AND maybe if you need:
`chat` (
`id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`message_id` int(11) UNSIGNED NOT NULL,
PRIMARY KEY (`id`),
);