MySQL Query Optimization for large tables - mysql

Question

I have a query that take 50 seconds
SELECT `security_tasks`.`itemid` AS `itemid`
FROM `security_tasks`
INNER JOIN `relations` ON (`relations`.`user_id` = `security_tasks`.`user_id` AND `relations`.`relation_type_id` = `security_tasks`.`relation_type_id` AND `relations`.`relation_with` = 3001 )
Records in security_tasks = 841321 || Records in relations = 234254
CREATE TABLE `security_tasks` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) DEFAULT NULL,
`itemid` int(11) DEFAULT NULL,
`relation_type_id` int(11) DEFAULT NULL,
`Task_id` int(2) DEFAULT '0',
`job_id` int(2) DEFAULT '0',
`task_type_id` int(2) DEFAULT '0',
`name` int(2) DEFAULT '0'
PRIMARY KEY (`id`),
KEY `itemid` (`itemid`),
KEY `relation_type_id` (`relation_type_id`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1822995 DEFAULT CHARSET=utf8;
CREATE TABLE `relations` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) DEFAULT NULL,
`relation_with` int(11) DEFAULT NULL,
`relation_type_id` int(11) DEFAULT NULL,
`manager_level` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
KEY `relation_with` (`relation_with`),
KEY `relation_type_id` (`relation_type_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1082882 DEFAULT CHARSET=utf8;
what can i do to make it fast, like 1 or 2 seconds fast
EXPLAIN :
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE relations ref user_id,relation_with,relation_type_id relation_with 5 const 169 Using where
1 SIMPLE security_tasks ref relation_type_id,user_id user_id 5 transparent.relations.user_id 569 Using where
UPDATE :
adding a composite key minimized the time to 20 seconds
ALTER TABLE security_tasks ADD INDEX (user_id, relation_type_id) ; ALTER TABLE relations ADD INDEX (user_id, relation_type_id) ; ALTER TABLE relations ADD INDEX (relation_with) ;
The problem is when the relations table has large data for the selected user (relations.relation_with` = 3001 )
any ideas ?

Answer 1

Adjust your compound index slightly, don't do just two, but all three parts
ALTER TABLE relations ADD INDEX (user_id, relation_type_id, relation_with)
The index does not just have to be on the joined columns, but SHOULD be based on joined columns PLUS anything else that makes sense as querying criteria is concerned (within reason, takes time to learn more efficiencies). So, in the case suggested, you know the join on the user and type, but are also specific to the relation with... so that is added to the same index.
Additionally, your security task table, you could add the itemID to the index to make it a covering index (ie: covers the join conditions AND the data element(s) you want to retrieve). This too is a technique, and should NOT include all other elements in a query, but since this is a single column might make sense for your scenario. So, look into "covering indexes", but in essence, a covering index qualifies the join, but since it also has this "itemid", the engine does not have to go back to the raw data pages of the entire security tasks table to get that one column. It's part of the index so it grabs whatever qualified the join and comes along for the ride and you are done.
ALTER TABLE security_tasks ADD INDEX (user_id, relation_type_id, itemid) ;
And for readability purposes, especially with long table names, it's good to use aliases
SELECT
st.itemid
FROM
security_tasks st
INNER JOIN relations r
ON st.user_id = r.user_id
AND st.relation_type_id = r.relation_type_id
AND r.relation_with = 3001