Individual queries run on 10ms, with UNION ALL they are taking 290ms+ (7.7M records MySQL DB). How to optimise?

I have a table that stores available appointments for teachers, allowing two kinds of insertions:

-
Hourly based: with total freedom to add unlimited slots per day per teacher (as long as slots don't overlap): on 15/Apr a teacher may have slots at 10:00, 11:00, 12:00 and 16:00. A person is served after choosing a specific teacher time/slot.

-
Time period/range: on 15/Apr another teacher may work from 10:00 to 12:00 and then from 14:00 to 18:00. A person is served by order of arrival, so if a teacher works from 10:00 to 12:00, all persons that arrive in this period will be attended by order of arrival (local queue).

Since I have to return all available teachers in a search, I need all slots to be saved in the same table as the order of arrival ranges. This way I can order by date_from ASC, showing the first available slots first on the search results.

Current table structure

CREATE TABLE `teacher_slots` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `teacher_id` mediumint(8) unsigned NOT NULL,
  `city_id` smallint(5) unsigned NOT NULL,
  `subject_id` smallint(5) unsigned NOT NULL,
  `date_from` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `date_to` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `status` tinyint(4) NOT NULL DEFAULT '0',
  `order_of_arrival` tinyint(1) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `by_hour_idx` (`teacher_id`,`order_of_arrival`,`status`,`city_id`,`subject_id`,`date_from`),
  KEY `order_arrival_idx` (`order_of_arrival`,`status`,`city_id`,`subject_id`,`date_from`,`date_to`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

Search query

I need to filter by: actual datetime, city_id, subject_id and if a slot is available (status = 0).

For hourly based I have to show all available slots for the first closest available day for every teacher (show all time slots of a given day and can't show more than one day for the same teacher). (I got the query with the help from mattedgod).

For range based (order_of_arr

Try this query:

(
select * from (SELECT id, teacher_slots.teacher_id, date_from, date_to,  order_of_arrival
FROM teacher_slots  WHERE teacher_slots.date_from >= '2014-04-10 08:00:00'
    AND teacher_slots.order_of_arrival = 0
    AND teacher_slots.status = 0
    AND teacher_slots.city_id = 6015
    AND teacher_slots.subject_id = 1) 
 teacher_slots
JOIN (
    SELECT DATE(MIN(date_from)) as closestDay, teacher_id
    FROM teacher_slots
    WHERE   date_from >= '2014-04-10 08:00:00' AND order_of_arrival = 0
            AND status = 0 AND city_id = 6015 AND subject_id = 1
    GROUP BY teacher_id
) a ON a.teacher_id = teacher_slots.teacher_id
AND DATE(teacher_slots.date_from) = closestDay

)

UNION ALL

(
SELECT id, teacher_id, date_from, date_to, order_of_arrival
FROM teacher_slots
WHERE order_of_arrival = 1 AND status = 0 AND city_id = 6015 AND subject_id = 1
    AND (
        (date_from = '2014-04-10 08:00:00')
        OR (date_from >= '2014-04-10 08:00:00')
    )
GROUP BY teacher_id
)

ORDER BY date_from ASC;

StackExchange Database Administrators Q#39764, answer score: 2