MySql Query Optimization with LEFT JOIN - mysql

Question

I have a MySQL query to optimized because when this query runs it increase MySQL process CPU usage to 100%. So I did some modification on that query and now it decrease its running time but still CPU usage is near to 100%. I index all search fields in both tables.
These 'milestone_and_events' and 'milestone_and_events_reminders' has one to many relationship ('milestone_and_events' have one or more 'milestone_and_events_reminders') and 'milestone_and_events_reminders' table have nearly 1.5 million records.
Is there any way to further optimize this query, can any one give me a guide ?? thank you
This is original query
SELECT MAE.*
, MAER.reminder_user_id
, MAER.reminder_type
, MAER.id as reminder_id
FROM milestone_and_events AS MAE
LEFT
JOIN milestone_and_events_reminders MAER
ON MAE.id = MAER.milestone_and_events_id
WHERE MAER.alert_time < '$currentTime'
AND MAER.issued ! = 1
AND MAE.completed = 0
GROUP
BY MAE.id
, MAER.reminder_user_id
, MAER.reminder_type
This is my current query
$currentTime = date('Y-m-d H:i:s');
$query = 'SELECT MAE.id, MAE.time, MAER.reminder_user_id, MAER.reminder_type, MAER.id AS reminder_id
FROM milestone_and_events AS MAE
LEFT JOIN milestone_and_events_reminders MAER ON
MAE.id = MAER.milestone_and_events_id AND
MAER.issued =0 AND
MAER.alert_time < "' . $currentTime . '" AND
MAE.completed =0
WHERE MAER.reminder_type != "onTime"
GROUP BY MAER.milestone_and_events_id,MAER.reminder_user_id,MAER.reminder_type';
UPDATE 1
this 'milestone_and_events' table have nearly 200 entries.
CREATE TABLE `milestone_and_events` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`description` varchar(250) NOT NULL,
`time` datetime NOT NULL,
`attached_type` int(11) DEFAULT NULL,
`attached_type_value` int(11) DEFAULT NULL,
`completed` int(11) NOT NULL DEFAULT '0',
`type` int(11) NOT NULL DEFAULT '1',
PRIMARY KEY (`id`),
KEY `time` (`time`),
KEY `completed` (`completed`),
KEY `id` (`id`,`completed`)
) ENGINE=InnoDB AUTO_INCREMENT=154 DEFAULT CHARSET=utf8
'milestone_and_events_reminders' table have nearly 1.5 million entries.
CREATE TABLE `milestone_and_events_reminders` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`reminder_user_id` int(11) NOT NULL,
`milestone_and_events_id` int(11) NOT NULL,
`alert_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`issued` tinyint(11) NOT NULL DEFAULT '0',
`reminder_type` enum('upComming','delayed','onTime') NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`reminder_user_id`),
KEY `reminder_type` (`reminder_type`),
KEY `issued` (`issued`),
KEY `milestone_and_events_id` (`milestone_and_events_id`, `issued`,`reminder_type`),
CONSTRAINT `milestone_and_events_reminders_ibfk_1` FOREIGN KEY (`milestone_and_events_id`)
REFERENCES `milestone_and_events` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=3323544 DEFAULT CHARSET=utf8

Answer 1

This is your query:
SELECT MAE.id, MAE.time, MAER.reminder_user_id, MAER.reminder_type, MAER.id AS reminder_id
FROM milestone_and_events AS MAE LEFT JOIN
milestone_and_events_reminders MAER
ON MAE.id = MAER.milestone_and_events_id AND
MAER.issued = 0 AND
MAER.alert_time < "' . $currentTime . '"
WHERE MAE.completed = 0 AND MAER.reminder_type <> "onTime"
GROUP BY MAE.id, MAER.reminder_user_id, MAER.reminder_type;
Note: I moved the condition MAE.completed = 0 from the on clause to the where clause. It does nothing in the on clause. And, I changed the first key in the group by to match the SELECT.
The best indexes for this query are composite indexes : milestone_and_events(completed, id) and milestone_and_events_reminders(milestone_and_events_id, issued, alert_time, reminder_type).
My guess is that putting the completed = 0 where it belongs will reduce the amount of data and improve the performance of the query.