MySQL query with multiple joins taking too long to execute - mysql

Question

I have 3 tables. The first one is called map_life, the second one is called scripts and the third one is called npc_data.
I'm running the following query to get all the properties from map_life while also getting the script column from scripts and the storage_cost column from npc_data if the ids match.
SELECT life.*
, script.script
, npc.storage_cost
FROM map_life life
LEFT
JOIN scripts script
ON script.objectid = life.lifeid
AND script.script_type = 'npc'
LEFT
JOIN npc_data npc
ON npc.npcid = life.lifeid
As you can see, map_life id is lifeid, while scripts id is objectid and npc_data id is npcid.
This query is taking about 5 seconds to execute, and I have no idea why. Here's the CREATE statements for all those 3 tables, maybe I'm missing something?
CREATE TABLE `mcdb83`.`map_life` (
`id` bigint(21) unsigned NOT NULL AUTO_INCREMENT,
`mapid` int(11) NOT NULL,
`life_type` enum('npc','mob','reactor') NOT NULL,
`lifeid` int(11) NOT NULL,
`life_name` varchar(50) DEFAULT NULL COMMENT 'For reactors, specifies a handle so scripts may interact with them; for NPC/mob, this field is useless',
`x_pos` smallint(6) NOT NULL DEFAULT '0',
`y_pos` smallint(6) NOT NULL DEFAULT '0',
`foothold` smallint(6) NOT NULL DEFAULT '0',
`min_click_pos` smallint(6) NOT NULL DEFAULT '0',
`max_click_pos` smallint(6) NOT NULL DEFAULT '0',
`respawn_time` int(11) NOT NULL DEFAULT '0',
`flags` set('faces_left') NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `lifetype` (`mapid`,`life_type`)
) ENGINE=InnoDB AUTO_INCREMENT=32122 DEFAULT CHARSET=latin1;
CREATE TABLE `mcdb83`.`scripts` (
`script_type` enum('npc','reactor','quest','item','map_enter','map_first_enter') NOT NULL,
`helper` tinyint(3) NOT NULL DEFAULT '-1' COMMENT 'Represents the quest state for quests, and the index of the script for NPCs (NPCs may have multiple scripts).',
`objectid` int(11) NOT NULL DEFAULT '0',
`script` varchar(30) NOT NULL DEFAULT '',
PRIMARY KEY (`script_type`,`helper`,`objectid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Lists all the scripts that belong to NPCs/reactors/etc. ';
CREATE TABLE `mcdb83`.`npc_data` (
`npcid` int(11) NOT NULL,
`storage_cost` int(11) NOT NULL DEFAULT '0',
`flags` set('maple_tv','is_guild_rank') NOT NULL DEFAULT '',
PRIMARY KEY (`npcid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Answer 1

For this query:
SELECT l.*, s.script, npc.storage_cost
FROM map_life l LEFT JOIN
scripts s
ON s.objectid = l.lifeid AND
s.script_type = 'npc' LEFT JOIN
npc_data npc
ON npc.npcid = l.lifeid;
You want indexes on: scripts(object_id, script_type, script) and npc_data(npcid, storage_cost). The order of the columns in these indexes is important.

Answer 2

map_life.lifeid does not have any indexes defined, therefore the joins will result in full table scans. Define an index on map_life.lifeid field.
In scripts table the primary key is defined on the following fields in that order: script_type, helper, objectid. The join is done on objectid and there is a constant filter criterion on script_type. Because the order of the fields in the index is wrong, MySQL cannot use the primary key for this query. For this query the order of the fields in the index should b: objectid, script_type, helper.
The above will significantly speed up the joins. You may further increase the speed of the query if your indexes actually cover all fields that are in the query because in this case MySQL does not even have to touch the tables.
Consider adding an index with the following fields and order to the scripts table: object_id, script_type, script and npcid, storage_cost index to npc_data table. However, these indexes may slow down insert / update / delete statements, so do some performance testing before adding these indexes to production environment.